,RTH :NCES RARY pale 'Bicentennial STUDIES IN EVOLUTION gale 'Bicentennial publication?! With the approval of the President and Fellows of 1C ale University, a series of volumes has been prepared by a number of the Professors and In- structors, to be issued in connection with the Bicentennial Anniversary, as a partial indica- tion of the character of the studies in which the University teachers are engaged. / This series of volumes is respectfully dedicated to of tlje STUDIES IN EVOLUTION MAINLY REPRINTS OF OCCASIONAL J PAPERS^ ' SELECTED FROM THE PUBLICATIONS OF THE LABORATORY OF INVERTEBRATE PALEONTOLOGY, PEABODY MUSEUM YALE UNIVERSITY BY CHARLES EMERSON BEECHER NEW YORK: CHARLES SCRIBNER'S SONS LONDON: EDWARD ARNOLD 1901 EARTH CIENC UBRARY Copyright, 1901, BY YALE UNIVERSITY Published^ August, IQOI EARTH UNIVERSITY PRESS JOHN WILSON AND SON CAMBRIDGE, U.S.A. PREFACE THE following papers from the publications of the Labora- tory of Paleontology have been selected for reprinting on account of their representing more or less closely a distinct line of research; namely, the investigation and study of the development of a number of invertebrate animals. The gen- eralizations resulting from such studies properly belong to the province of organic evolution, while the detailed methods pertain to the observation and interpretation of the stages of growth and decline in the organism. Nearly all the subject-matter i^ based upon studies of the remains of fossil animals, many of them coming from the oldest known fossil-bearing rocks. In some instances material representing living species has been introduced for comparison and to illustrate further the problems under investigation. The first work done in America on the stages of growth of fossil Brachiopoda was a memoir by the present writer, in collaboration with Dr. John M. Clarke, on " The Development of Some Silurian Brachiopoda," published by the University of the State of New York. It seems fitting to include this, in order to complete the work on the development of the Brachiopoda carried on subsequently by the writer. Another joint paper, written in connection with Mr. Charles Schuchert, is also introduced. The first paper in the present collection, on " The Origin and Significance of Spines," is an attempt to apply the general laws of evolution in the study of a particular structure throughout 938005 Vlll PREFACE the animal and vegetable kingdoms, and to discover its true in- terpretation in terms of ontogeny, phylogeny, and chronology. The next division of the volume comprises papers on the structure and development of Trilobita. The following section presents developmental studies on the Brachiopoda. It should be stated that this work was undertaken largely in the hope that the results would lead to the principles governing a natural classification of all forms in these two classes. In the brachio- pods nothing further than a division into orders and a grouping of the families under the orders was attempted. The elabora- tion of this classification has been very fully carried out by Mr. Charles Schuchert, in his " Synopsis of American Brachiop- oda." For the Trilobita, a new arrangement into orders was suggested, together with a redefining of the families and a grouping of the genera under them. In the last division are three papers on special problems of development. The author is greatly indebted to Miss Lucy P. Bush for assistance in arranging the material for this volume, and especially for aid while it was being printed. YALE UNIVERSITY, April 3, 1901. CONTENTS PAGE I. GENERAL EVOLUTION 1. THE ORIGIN AND SIGNIFICANCE OF SPINES 3 Introduction 3 Law of Variation 5 Definition of Terms 9 Growth of a Spine 10 Localized Stages of Growth 14 Compound Spines 16 Application of Law of Morphogenesis 17 Ontogeny of a Spinose Individual 18 Phylogeny of Spinous Forms 23 Categories of Origin 26 Conditions or Forces affedting Growth 31 A. External Stimuli 32 B. Growth Force . . ... > . .... 34 C. External Restraint 38 D. Deficiency of Growth Force 39 Summary of Causes of Spine Genesis 41 I. In response to stimuli from the environment act- ing on most exposed parts. (Aj.) .... 42 II. As extreme results of progressive differentiation of previous structures. (A 2 , B 3 .) . . . . 47 III. Secondarily as a means of protection and offence. (A ft B 4 .) 52 IV. Secondarily from sexual selection. (A 4 , B 4 .) . 57 V. Secondarily from mimetic influences. (A 6 , B 4 .) . 60 VI. Prolonged development under conditions favor- able for multiplication. (B!.) 64 VII. By repetition. (B 2 .) 67 VIII. Restraint of environment causing suppression of structures. (C x .) 70 IX. Mechanical restraint. (C 2 .) 77 X. Disuse. (C 3 , D 2 .) 80 XI. Intrinsic suppression of structures and functions. D. 86 x CONTENTS PAGE 1. THE ORIGIN AND SIGNIFICANCE OF SPINES Con- tinued Categories of Interpretation 93 Spinosity a Limit to Variation 93 Spinosity the Paracme of Vitality 97 Conclusion 99 References 102 II. STRUCTURE AND DEVELOPMENT OF TRILOB1TES 1. OUTLINE OF A NATURAL CLASSIFICATION OF THE TRILOBITES 109 Introduction 109 Previous Classifications 110 Rank of the Trilobites 114 Comparative Morphology of Crustacea 115 Morphology of the Cephalon ...117 Principles of a Natural Classification 119 Application of Principles for Ordinal Divisions . . . 121 Application of Principles for Arrangement of Families and Genera 125 Diagnoses and Discussions 130 Arrangement of the Families of Trilobites 131 Diagnoses and Discussions of Orders and Families . . 134 Hypoparia 134 Family I. Agnostidas . 135 Family II. Harpedidae 137 Family III. Trinucleidaa 138 Opisthoparia 138 Family IV. Conocoryphidae 140 Family V. Olenidas 141 I. Paradoxinse 143 II. Oryctocephalinae 145 III. Oleninae 145 IV. Dikelocephalinae 145 Family VI. Asaphidse 145 I. Asaphidas 146 II. Illsenidae ' 146 Family VII. Proetidse 147 Family VIII. Bronteidae 149 Family IX. Lichadidse 150 Family X. Acidaspidas 151 Proparia 152 Family XI. Encrinuridse 153 Family XII. Calymmenidae 154 CONTENTS xi PAGE 1. OUTLINE OF A NATURAL CLASSIFICATION OF THE TRILOBITES Continued Family XIII. Cheiruridse 155 Family XIV. Phacopidse ....... 15 References 157 List of Genera 159 2. THE SYSTEMATIC POSITION OF THE TRILOBITES . . 163 8. THE LARVAL STAGES OF TRILOBITES 166 Introduction 166 The Protaspis 167 Review of Larval Stages of Trilobites 171 Analysis of Variations in Trilobite Larvse 179 Antiquity of the Trilobites 183 Restoration of the Protaspis 185 The Crustacean Nauplius 188 Summary 193 References 195 4. ON THE MODE OF OCCURRENCE, AND THE STRUCTURE AND DEVELOPMENT OF TRIARTHRUS BE OKI . . 197 5. FURTHER OBSERVATIONS ON THE VENTRAL STRUC- TURE OF TRIARTHRUS 203 Paired Uniramose Appendages 205 Anterior Antennae, or Antennules 205 Paired Biramous Appendages 205 First Pair of Biramous Appendages, or Posterior Antennae 206 Second Pair of Biramous Appendages, or Mandibles 206 Third and Fourth Biramous Appendages, or Maxillae 206 Thoracic Legs 207 Organs in the Median Line 208 The Hypostoma 208 The Mouth 209 The Metastoma 209 The Anal Opening 209 Observations 210 Summary of Ventral Organs of Triarthrus . . . . 211 6. THE MORPHOLOGY OF TRIARTHRUS 213 References 219 7. STRUCTURE AND APPENDAGES OF TRINUCLEUS . . . 220 Appendages 223 Endopodites 224 Exopodites 224 xii CONTENTS PAGE III. STUDIES IN THE DEVELOPMENT OF THE BRACH- IOPODA 1. DEVELOPMENT OF THE BRACHIOPODA 229 I. Introduction 229 The Protegulum 230 Affinities . 232 Modifications from Acceleration 233 Differences in the Valves 234 Genesis of Form 238 Types of Pedicle-openings . 240 Atremata 243 Neotremata . 244 Protremata 244 Telotremata 245 II. Classification of the Stages of Growth and Decline . 246 Embryonic Stages 247 Larval Stages 250 Origin of the Deltidium and Deltidial Plates . . 257 Post-embryonic Stages 265 Nepionic Period 267 Neanic Period 268 Ephebic Period 269 Gerontic Period 269 Synopsis 271 References 272 III. Morphology of the Brachia 274 Classification of Brachial Structures .... 276 Leiolophus Stage 277 Taxolophus Stage 277 Trocholophus Stage 278 Schizolophus Stage 278 Ptycholophus Stage 280 Zugolophus and Plectolophus Stages . . . 281 Spirolophus Stage 282 References 285 2. SOME CORRELATIONS OF ONTOGENY AND PHYLOGENY IN THE BRACHIOPODA 286 3. REVISION OF THE FAMILIES OF LOOP-BEARING BRACH- IOPODA 290 The Terebratulidae 290 The Terebratellidae 291 Magellaniinae 293 Dallininae 295 Comparisons and Homologies 299 Morphogeny from Gwynia to Megathyris 302 CONTENTS xiii PAGE 3. REVISION OF THE FAMILIES OF LOOP-BEARING BRACHI- OPODA Continued Morphogeny from Gwynia to Dallina 303 Morphogeny from Gwynia to Magellania 303 Conclusions 304 Classification 305 Family Terebratulidse Gray 306 Centronellinae Waagen 306 Stringocephalinae Dall 307 Terebratulinae Dall 307 Dyscoliinae (=Dyscoliid8e Fischer and (Ehlert emend.) 307 Family Terebratellidae King emend 307 Dallininae n. sub.-fam 307 Magellaniinse n. sub.-fam 308 Megathyrinse Dall 308 References 308 4. DEVELOPMENT OF SOME SILURIAN BRACHIOPODA . . 310 Introduction 310 List of the Brachiopoda occurring in the Niagara Shales at Waldron, Indiana 314 Discussions of the Species 317 Crania siluriana Hall,, 1863 317 Dalmanella elegantula Dalman, 1827 317 Specific Characters 318 Mature Form 318 Incipient Form 319 Developmental Changes 320 General Form and Outline 320 Beaks 320 Foramen 320 Plications 321 RJiipidomella hybrida Sowerby, 1839 321 Leptcena rhomboidalis Wilckens, 1769 322 Specific Characters 323 Mature Form 323 Incipient Form 323 Developmental Changes 324 Development of Leptcena rhomboidalis . . . 325 Orthothetes subplanus Conrad, 1842 327 Specific Characters 328 Mature Form 328 Incipient Form 328 Developmental Variations 329 Strophonella striata Hall, 1843 330 Specific Characters 331 xiv CONTENTS PAGE 4. DEVELOPMENT OF SOME SILURIAN BRACHIOPODA Con- tinued Mature Form 331 Incipient Shell 331 Developmental Changes 331 Mimulus waldronensis Miller and Dyer, 1878 . . 334 Dictyonella reticulata Hall, 1868 335 Anastrophia internascens Hall, 1879 337 Specific Characters 337 Mature Form 337 Incipient Form 338 Developmental Changes 338 Camarotcechia acinus Hall, 1863 339 Specific Characters 340 Mature Form 340 Variations from the Normal Adult .... 340 Initial Shell 340 General Developmental Characters 341 Camarotcechia neglecta Hall, 1852 341 Specific Characters 342 Mature Form 342 Incipient Form 343 Developmental Variations 343 General Form and Outline 343 Beak and Foramen 343 Plications 343 Camarotcechia Whitii Hall, 1863 344 Specific Characters 344 Mature Form 344 Abnormalities at Maturity 345 Incipient Form 345 Developmental Variations 345 General Form and Outline 345 Beak and Foramen 346 Plications 346 Camarotcechia indianensis Hall, 1863 346 Specific Characters 347 Normal Mature Form 347 Variations from the Normal 348 A. Forms with one plication in the ven- tral sinus 348 B. Forms with three plications in the ventral sinus . . 348 C. Forms with four plications in the ven- tral sinus 348 Monstrous Forms 348 Initial Shell 349 CONTENTS xv PAGE 4. DEVELOPMENT OF SOME SILURIAN BRACHIOPODA Con- tinued Developmental Variations 349 General Form and Outline 349 Beak 350 Foramen 350 Plications 351 Rhynchotreta cuneata Dalinan, 1827, var. americana Hall, 1879 351 Specific Characters 352 Mature Form 352 Incipient Form 353 Developmental Changes 353 Contour 353 Fold and Sinus 353 Beak 354 Surface Ornaments 354 Cardinal Area 355 Variations 356 Atrypa reticularis Linnaeus, 1767 356 Specific Characters 356 Mature Form 356 Incipient Form .'......... 357 Developmental Variations 358 General Form and Outline 358 Beak 358 Foramen , . . . 358 Plications 359 Summary 359 Homceospira evax Hall, 1863 360 Specific Characters 360 Mature Form 360 Variations from the Normal Development . 361 Developmental Variations 362 Beaks 363 Foramen 363 Sinus 364 Plications 364 Internal Apparatus 364 Homceospira sobrina sp. nov 366 Specific Characters 367 Mature Form 367 Variations from the Normal Mature Form . 368 Incipient Form 368 Developmental Variations 368 CONTENTS PAGE 4. DEVELOPMENT OF SOME SILURIAN BRACHIOPODA Con- tinued General Form and Outline 368 Beak and Foramen 369 Atrypina disparilis Hall, 1852 369 Specific Characters 370 Mature Form 370 Variations in Outline 370 Abnormalities 370 Developmental Changes 371 Meristina rectirostris Hall, 1882 372 Specific Characters 373 Mature Form 373 Incipient Form 373 Developmental Variations 374 General Form and Outline ...... 374 Beak 374 Foramen 374 Whitfieldella nitida Hall, 1843 374 Specific Characters 375 Mature Form 375 Variations in Outline 375 Incipient Form 376 Developmental Variations 376 Meristina Maria Hall, 1863 377 Specific Characters 378 Mature Form 378 Incipient Form 379 Developmental Variations 379 General Form and Outline 379 Beak 379 Foramen 379 Spirifer crispus Hisinger, 1826 380 Spirifer crispus, var. simplex Hall, 1879 .... 380 Reticularia bicostata Vanuxem, 1842, var. petila Hall, 1879 380 Spirifer radiatus Sowerby, 1825 382 Incipient Form 382 Developmental Changes 383 Summary of Developmental Changes 386 Size and Contour 386 Valves 388 Beaks 389 Cardinal Area 389 Internal Apparatus 395 Surface Ornaments ......... 396 Varieties and Abnormalities . 397 CONTENTS xvii PAGE 5. DEVELOPMENT OF BILOBITES 399 Developmental Changes in BUobites various .... 402 Observations 404 6. DEVELOPMENT OF TEREBRATALIA OBSOLETA DALL . . 7. DEVELOPMENT OF THE BRACHIAL SUPPORTS IN DIE- LASMA AND ZYGOSPIRA 410 Development of the Loop in Dielasma turgidum . . . 412 Development of the Brachial Supports in Zygospira recurvirostris 413 Observations and Correlations 415 IV. MISCELLANEOUS STUDIES IN DEVELOPMENT 1. DEVELOPMENT OF A PALEOZOIC PORIFEROUS CORAL . . 421 Development of Pleurodictyum lenticular e 422 General Conclusions 425 2. SYMMETRICAL CELL DEVELOPMENT IN THE FAVOSITID^E 429 Summary 433 3. DEVELOPMENT OF THE SHELL IN THE GENUS TOR- NOCERAS HYATT 435 V. PLATES AND EXPLANATIONS. 441 INDEX 597 ILLUSTRATIONS LIST OF PLATES I. Spines of Radiolaria. II. Classification of Trilobites. III. Larval Stages of Trilobites. IV. Larval Stages of Trilobites. V. Crustacean Larvae. VI. Appendages of Triarthrus. VII. Ventral Side of Triarthrus. VIII. Appendages of Triarthrus. IX. Triarthrus. X. Appendages of Trinucleus. XI. Stages of Growth of Brachiopoda. XII. Stages of Growth of Brachiopoda. XIII. Parallelism in Brachiopoda (Magellania Series). XIV. Ontogeny and Phylogeny of the Terebratellidae. XV. Stages of Growth in Silurian Brachiopoda. XVI. Stages of Growth in Silurian Brachiopoda. XVII. Stages of Growth in Silurian Brachiopoda. XVIII. Stages of Growth in Silurian Brachiopoda. XIX. Stages of Growth in Silurian Brachiopoda. XX. Stages of Growth in Silurian Brachiopoda. XXI. Stages of Growth in Silurian Brachiopoda. XXII. Stages of Growth in Silurian Brachiopoda. XXIII. Development of BiloUtes. XXIV. Development of Terebratalia. XXV. Development of Terebratalia. XXVI. Brachial Supports in Dielasma and Zygosplra. XXVII. Development of Pleurodictyum. XXVIII. Pleurodictyum. XXIX. Pleurodictyum. XXX. Pleurodictyum. XXXI. Pleurodictyum and Favosites. XXXII. Favositidaj. XXXIII. Favositidse. XXXIV. Tornoceras. xx ILL US TRA TIONS FIGURES IN TEXT FIGURE PAGE 1-5. Different stages of growth of a spine 10 6. A profile of a single radiating ridge of Spondylus princeps ; showing the series of flattened spines 10 7-12. Diagrams ; showing growth and differentiation of ornament into spines . 11 13. Summer shoot of Barberry ; showing the gradations between leaves and spines 12 14. Profile of one of the primary rays of Spondylus imperialis ; show- ing the series of spines 12 15. Example of spine growth of simple increscence 13 16. Stages of spine growth by successive replacement .... . 13 17. Stages of spine growth by serial repetition 13 18. Stages of spine growth by decrescence 13 19. Sector; showing in diagram the multiplication of radiating lines by interpolation 15 20. Profiles of spines produced on the various radii at the four zones; as indicated in the preceding figure 15 21. Simple spine 16 22. Spine, with lateral spinules 16 23. Spine, with forked apex and lateral spinulose spinules ... 16 24. Prodissoconch of Ostrea virginiana . 20 25. Each stage of Avicula sterna 20 26. Young of Avicula sterna ; showing the beginning of spine growth 20 27. Young Saxicava arctica 20 28. Young Anomia aculeata 20 29. Young Spondylus princeps 20 30. Side view of Spondylus calcifer ; about one-third grown ; show- ing the characteristic spinous growth 21 31. Side view of Spondylus calcifer ; showing the greatly thickened right valve and the entire absence of spines over the whole shell 21 32. Attlieya decora, a diatom, with spines from the angles .... 44 33. Difflugia acuminata, a freshwater rhizopod; showing spiniform projection of the fundus 44 34. Difflugia constricta, a freshwater rhizopod, with rounded fundus 44 35. The same; showing a single spine on the fundus 44 36. The same; showing two spines 44 37. Cyaihophycus reticulatus. Ordovician 49 38. Dictyospongia Conradi. Devonian 49 39. Hydroceras tuberosum. Devonian 49 ILL USTRA T10NS xxi FIGURE PAGE 40. Lima squamosus 51 41. Antler of Cervulus (?) dicranoceras 53 42. Antler of Cervus pardinensis - JJ3L 43. Antler of the Fallow Deer (Cervus dama) 53 44. Zoe'a of the common crab (Cancer irroratus) 56 45. Profile of head of Chamceleon Oweni ; male 60 46. Female of the same species 60 47. Profile of a spider (Ccerostris mitralis) on a twig mimicking a spiny excrescence 61 48. The larva of the Early Thorn Moth (Selenia illunarid) resting on a twig ; showing mimicry of stem and spiniform processes 61 49. Australian Pipe-fish (Phyllopteryx eques) and frond of sea-weed in lower right-hand corner; showing mimicry 62 50. Allorchestes armatus, a spiny amphipod from Lake Titicaca; female 65 51. Acontaspis hastata, a radiolarian ; showing multiplication of spines by repetition 69 52. Strophalosia keokuk, an attached brachiopod; showing the spines extending from the ventral valve to and along the surface of attachment 69 53. A gastropod shell (Platyceras) to which are attached a number of Strophalosia keokuk 69 54. The spiny Cytlsus (C. spinosus); showing suppression of branches into spines 75 55. A single leaf of Tragacanth (Astragalus Tragacantha), from which the three upper leaflets have fallen 75 56. Leaf axis of the same, from which all the leaflets have fallen . . 75 57. Twig of common locust (Robinia Pseudacacia) ; showing spines representing stipules 75 58. Portion of skin of Python ; showing the spurs which represent the suppressed or vestigial hind legs 76 59. Bones of suppressed legs of Python 76 60. Dorsal view of Spirifer mucronatus ; Devonian ; showing spini- form cardinal angles 78 61. Illcenus (Octillcenus) Hisingeri, Ordovician, Bohemia; a trilobite; showing spiniform pleural extremities of first thoracic segment 79 62. Cheirurus insignis, Silurian, Bohemia ; pygidium and six thoracic segments 79 63. Deiphon Forbesi, Silurian, Bohemia ; entire specimen; showing spiniform pleura of segments corresponding in direction to those of the pygidium 79 64. Lichas scabra, Silurian, Bohemia ; pygidium, with three thoracic segments ; showing spiniform ends of pleura 79 X xii ILL USTRA TIONS FICMJRK PAGE 65. Paradoxides spinosus. Cambrian, Bohemia ; pygidium and six free segments 79 66. Female of Lernceascus nematoxys, a parasitic copepod ; showing suppression of limbs 85 67. Horse-shoe Crab (Limulus polyphemus) ; showing telson spine and abbreviated abdomen 85 68. A Devonian phyllocarid (Echinocaris socialis) ; showing spini- form telson and cercopods 85 69. Wing of Apteryx australis 85 70. Skeleton of right fore limb of the Jurassic Dinosaur Iguanodon bernissartensis ; showing suppressed first digit 85 71. Leaf of Ratan (Dcemonorops hygrophilus) 89 72. Leaf of Ratan (Desmoncus polycanihus) 89 73. Bramble (Rubus squarrosus^) 89 74. Diagram and table ; showing correlation of stages and conditions of development in the spinose individual, in its ancestry, and in time 100 75. Table of geological distribution of Trilobita 133 76. Agnostus nudus Beyrich 177 77. Agnostus rex Barrande 177 78. Trinucleus ornatus Sternberg 177 79. Hydrocephalus saturnoides Barrande 177 80. Hydrocephalus carens Barrande 177 81-83. Olenellus (Mesonacis) asaphoides Emmons 177 84. Geological range and distribution of Arthropoda 184 85-94. Cistella neapolitana Scacchi 248-250 95-98. SpirorUs borealis Daudin 253 99, 100. Cistella neapolitana Scacchi 254 101-107. Thecidium (Lacazella) mediterraneum Risso 258 108-113. Cistella neapolitana Scacchi .... 261 114. Delthyrium of young Rhynchonella, without deltidial plates . . 262 115. The same at a later stage, with two triangular deltidial plates . 262 116. The same after completed growth ; showing joining of deltidial plates, and limitation of pedicle-opening to ventral beak . 262 117. Dorsal view of Magellania flavescens ; showing completed deltidial plates (del) 262 118. The same; profile 262 119. Dorsal view of umbonal portion of adult Terebratulina septen- trionalis, with shell removed by acid ; showing slight sec- ondary extension of ventral mantle around pedicle . . . 262 120. Dorsal view of umbonal portion of Magellania flavescens, with the shell removed by acid ; showing the complete envelop- ment of base of pedicle by secondary expansions from ven- tral mantle, and consequent production of deltidial plates filling delthyrium except at pedicle-opening 262 ILL US TRA TIONS xxiii FIGURE PAGE 121. Stages of growth of the lophophore in Thecidea, Cistella, and Megathyris 279 122. Stages of growth of the lophophore in the Terebratellidse and Terebratulidae 280 123. Metamorphoses of the brachidium in Dielasma turgidum . . . 281 124. Early stages of lophophore of Glottidia and adult brachia in Lingula and Hemithyris 282 125. Metamorphoses of brachidium of Zygospira and adult brachid- ium of Rhynchospira 284 126-128. Development of internal apparatus in Homceospira evax . 365 129. Deltidial development in Spirifer 384 130. Deltidial development in 1, 2, Spiriferina pinguis Deslong- champs ; 3, 4, Spiriferina Walcotti Sowerby; 5, Spiriferina rostrata Schlotheim 393 131. BiloUtes varicus Conrad ; ventral area 402 132. Genesis of Bilobites ... 404 I GENERAL EVOLUTION 1. THE ORIGIN AND SIGNIFICANCE OF SPINES i STUDIES IN EVOLUTION GENERAL EVOLUTION V A L i FO 1. THE ORIGIN AND SIGNIFICANCE OF SPINES A STUDY IN EVOLUTION* (PLATE I) INTRODUCTION THE presence of spines in various plants and animals is, at times, most obvious to all mankind, and not unnaturally they have come to be regarded almost wholly in the light of defensive and offensive weapons. Their origin, too, is commonly explained as due to the influence of natural selec- tion, resulting in the greater protection enjoyed by spiniferous organisms. But when, upon critical examination, it is seen that some animals are provided with spines which apparently interfere with the preservation of the individual, that other animals develop spines which cannot serve any purpose for protection or otherwise, and that spines themselves are often degenerate or suppressed organs, then it becomes evident that the spinose condition may have other interpretations than the single one of protection. The object of this article is to make a few observations on spinosity, especially among invertebrate animals, and to endeavor to arrive at some general conclusions relating to the origin and significance of this condition. It is believed that the results have a broader application than is at first apparent, and underlie important laws and principles of organic evolu- tion. In closely related species, the presence or absence of * Amer. Jour. Sci. (4), VI, 1-20, 125-136, 249-268, 329-359, pi. i, 1898. STUDIES IN EVOLUTION spines seems in itself a trivial character, indicating at best only specific differences, yet it will be shown that the spines are often the expression of important vital adjustments and conditions, and are not merely external features of the same value a& color and many other skin or superficial characters. As will be indicated later on, spines may also arise through 'the operations of a number of forces and conditions, and it may well be asked, therefore, Do spines have any profound significance? It must be granted at the outset that apart from other characteristics, or when regarded as simple spini- form extensions of certain tissues or organs, they have no such value or meaning. How, then, should they be con- sidered ? The reply is evident : Their importance lies not in what they are, but in what they represent. They are simply prickles, thorns, spines, or horns ; they represent, as will be shown, a stage of evolution, a degree of differentiation in the organism, a ratio of its adaptability to the environment, a result of selective forces, and a measure of vital power. After studying numerous organisms, the writer is led to believe that in every case no single reason is sufficient to account for this spinose condition. The original cause may not be operative through the entire subsequent phylogeny, so that spines arising from external stimuli and then serving important defensive purposes may at a later period practi- cally lose this function; or spines may become more and more developed simply by increasing diversity of growth forces, or through the multiplicity of effects. In this way causes may follow, overlap, or even coincide with each other ; but in interpreting special cases the problems involved may be quite complicated and often obscure. In reviewing the development of animal life from the earliest Cambrian to the present, one cannot avoid being impressed by the groups of spinose forms which appear here and there throughout geologic time, and give a special phase to contemporary faunas. Tracing these one by one through their geological development, it is noticed that each group began its history in small, smooth, or unornamented species. ORIGIN AND SIGNIFICANCE OF SPINES 5 As these developed, the spinose forms became more abundant until after the culmination of the group is reached, when this type either became extinct or was continued in smaller and less specialized forms. In applying this principle to any order of plants or animals, several precautions are necessary. The estimate must be based approximately upon the general average of the totality of specific characters, whether a genus, family, order, or even a class is being considered. A short- lived family or genus, or the terminal members of specialized groups, therefore, cannot be taken as representing the develop- mental status of the larger divisions, because they culminated and disappeared independently of the culmination of the class to which they belong. On a small scale, however, each epitomizes the rise and decline of the larger group, and the principles of correlation commonly applied in ontogeny and phylogeny can likewise be used in the study of spines and spiniferous species, with equally exact results, whenever the principal factors are understood. Law of Variation. Before undertaking any general or special examination of the life histories and interpretation of spinose organisms, it is desirable to consider briefly some of the biogenetic prin- ciples which are considered to bear directly on the problems here under discussion. First among these is the law of variation or change, which is so generally recognized as to require but the briefest restatement. The organic as well as the inorganic world is subject to all the forces of nature, internal and external, molecular and molar, and even a partial stability is gained only through a regulated adjustment. In organisms this change is momen- tary and persistent, while in most inorganic substances it is slow and intermittent. The results of this continual read- justment constitute modification, which may be progressive or regressive, continuous or discontinuous (in the sense of accelerated, uniform, or retarded). They are everywhere 6 STUDIES IN EVOLUTION present and the causes always operative. Throughout life the individual changes, and in addition varies from all other individuals. The family also changes with time, and like- wise differs from other families. Variation is everywhere present. Moreover, it is generally accepted, and is so taken here, that in its results this variation is not haphazard, but is normally in accordance with certain demands or in harmony with certain surroundings. Whether an organism itself tends to vary in all directions, or is chiefly subject to modifications from external forces, does not alter the preced- ing statement. Cope u has considered variation as either physico-chemical (molecular) or mechanical (molar). The influence of the first is known as physiogenesis and of the second as kineto- genesis. In the animal kingdom the potency of kinetogenesis is greater as an efficient cause of evolution; while in the vegetable kingdom physiogenesis is apparently of more importance. The tendency of variation is always in the direction of the establishment of an equilibrium between the organism and its environment. However, the laws of the development of the earth preclude the possibility of a constant environment, and therefore a perfect, permanent, and uniform equilibrium between life and surroundings is unattainable. The manner of variation is clearly defined as progressive and regressive. Progressive variation is one of the essential factors of evolution, while regressive variation is towards dissolution. Since the main history of life is told through processes of the former, progressive variation is far greater in importance ; while, in general, regressive variation can be applied only to late periods in the history of groups or forms now in their decadence, or to others which in past times have suffered decline and extinction. The summary of the operation of the law of multiplication of effects, as given by Herbert Spencer, 66 may well be stated here, as it emphasizes one of the principles through which spines have originated. ORIGIN AND SIGNIFICANCE OF SPINES 7 "It manifestly follows that a uniform force, falling on a uniform aggregate, must undergo dispersion ; that falling on an aggregate made up of unlike parts, it must undergo dis- persion from each part, as well as qualitative differentiations ; that in proportion as the parts are unlike, these qualitative differentiations must be marked; that in proportion to the number of the parts, they must be numerous; that the secondary forces so produced must undergo further trans- formations while working equivalent transformations in the parts that change them ; and similarly with the forces they generate. Thus the conclusions that a part-cause of evolu- tion is the multiplication of effects, and that this increases in geometrical progression as the heterogeneity becomes greater, are not only to be established inductively, but are deducible from the deepest of all truths." Modification, therefore, may properly include the results of the multiplication of effects. Furthermore, from a knowl- edge of the life history of the organic world, it is known that this change has been progressive, resulting in the evolution of the higher from the lower, of the complex from the simple, and of the definite from the indefinite. It must now be asked, Is the amount of variation with- out limit or is it restricted within bounds which can be determined? As far as can be seen, the limitations of the forms of species of animals and plants end only with the aggregate number of possibilities within the functional scope of the organism. Beyond, in either direction, is death, and a passage from the organic into the inorganic. The restric- tions of variation are chiefly those of temperature, pres- sure, motion, light, space, time, and matter. Within certain limits, these clearly bound the horizon of known possible life. Further, the material constitution of the organic world is naturally subject to ordinary mechanical and chemi- cal laws. If, instead of the preceding, general, and therefore rather abstract statements of the limits of variation, the subject is considered from the concrete, objective side, the limits 8 STUDIES IN EVOLUTION between which are found all the variations actually presented by any character or set of characters, in the animal or the vegetable kingdoms, can at once be determined. The fact that the organic world can be divided into kingdoms, sub- kingdoms, classes, orders, etc., and definitions of the divi- sions given, in itself furnishes sufficient evidence that these have been the limits of organic change, at least under present terrestrial conditions. This does not imply that the phylog- enies of groups of animals and plants do not converge and coalesce, and join larger and larger phyla in past ages, so that the gaps between unlike forms are gradually filled by complete series. It does, however, express the definite heterogeneity of the results of development. For the sake of illustrating an extreme range of variation, it will be granted that the assemblage of characters by which a mammal is now recognized precludes mammalian variation into a cold-blooded, non-vertebrate, lungless animal. Like- wise the mammalian skeleton cannot be siliceous or chitinous. Externally mammals may be smooth, hairy, scaly, or plated, but not feathered. There may be found numerous gradations from the smooth to the plated state, and a great range of variation in each type of epidermal structure. In vertebrate animals generally, the hair may vary in length, in fineness, in color and shape ; it may form bristles, or spines, or feathers ; and, as a skin character, it is related to horn-sheaths, hoofs, nails, claws, scales, and teeth. These constitute the limits of modification in epidermal or exoskeletal growths. The types are few, but the variety in each is almost infinite. The variation may be seen in individuals, but becomes greater in species, and increases still more in larger groups. The gradations are numerous between the hair of a Beaver and the spines of a Porcupine; between the horns of the Giraffe, Rhinoceros, and Antelope ; between the nails of Man and the claws of the Carnivora; and between the teeth of a Dog-fish and those of a Tiger. ORIGIN AND SIGNIFICANCE OF SPINES 9 Definition of Terms. In the beginning it is well to understand the meaning and extent of the terms included under the comprehensive word spine. In a general sense, spine is here used to cover any stiff, sharp-pointed process. A prickle is restricted in use to the small, sharp-pointed, conical projections which are purely cuticular; as in the Rose and Blackberry. A thorn is a sharp process on plants, usually representing a branch or stem. A horn is an excrescence on the head of cer- tain animals, and is properly hollow. An antler is a solid bony process, usually deciduous, and generally confined to the male; as in the Deer or Elk. A spur is a term applied to the claw-like process on the legs and wings of some birds, and on the hind legs of Ornithorhynchus and Echidna. The word spine, therefore, is most comprehensive, and is here intended to include the modified hairs of the Echidna and Porcupine ; the sharp, prickly scales of the Horned Toad (Phrynosoma); the pointed spiniform projections on the shells of Mollusca; the spinous prominences on the test of Crus- tacea and insects ; the fin spines as well as those on the oper- cula and scales of fishes ; the generally movable processes of Echinodermata ; the projecting rays and processes of Radio- laria, etc., etc. The vertebral column and also the processes from the separate vertebrae are known as spines, but as these are distinctly internal structures, they will not be considered in this connection. In nearly all classes of organisms spines have been devel- oped independently, and simply represent cases of parallel development of similar structures or morphological equiva- lents. They possess analogy of form without necessary homology of structure, and accordingly have no common phylogenetic connection. Therefore, if the relationships be- tween the smooth and spinose forms belonging to any group of animals or plants can be traced, and the simplest and most primitive condition in each case, as well as the highest stage of progressive development, can be ascertained, their relative 10 STUDIES IN EVOLUTION significance from an evolutionary standpoint may be confi- dently determined. Growth of a Spine. The growth of a spine is either direct an . progressive, or indirect and regressive. It is direct when it is developed by 12345 FIGURES 1-5. Different stages in the growth of a spine. 1, plane surface ; 2, slight elevation ; 3, node ; 4, short spine ; 5, completed simple spine. the addition of new tissue. In this way growth is attained in the antlers of a Deer, the horns of a Cow, the ordinary spines of Brachiopoda, Mollusca, and Crustacea, and in other similar examples covering the major- ity of cases. Growth is indirect, however, when the spine represents atrophy or suppression of an organ through the loss of its accessory parts ; as in the thorns of the Locust and the Barberry, the spiniform termination of the stems of the Pear, or the spurs on the Python. The direct development of a spine is essen- tially the same process in all cases. At a given point on the surface of an organism, there first appears a slight elevation, which becomes higher and higher, and is usually conical in form. This cone represents the simplest type of spine; and FIGURE 6. among animals and plants most spines conform a single radi- to this primitive pattern (figures 1-5). Often there are various kinds of surface orna- w- ments, which by growth and differentiation de- of g Ha^ned velop into spines. By rhythmic, alternating areas spines. O f accelerated or retarded growth, the concentric laminae on many molluscs may produce spines, as shown in figure 26. In the same way the radiating ridges may be ORIGIN AND SIGNIFICANCE OF SPINES 11 diversified into a row of spines, as represented in figure 6. Further, the surface may be reticulate, with longitudinal and transverse lines, and at the points of intersection, nodes and often spines are formed after the manner shown in figures 7-12. The longitudinal or vertical lines may become obsolete, leaving the spines to be borne on the transverse or 10 11 I , t j { i f i ( i i / f\ , , i 12 / y y / y / y y y / 9 i t.n ( y y / i lint FIGURES 7-12. Diagrams ; showing growth and differentiation of ornament into spines. 7, surface with parallel lines ; 8, surface with regular reticulate lines ; 9, same, with spines developed at the points of intersection ; 10, same, with the vertical lines obsolete, but still represented by the vertical rows of spines; 11, same, with the horizontal lines obsolete, but still represented by the horizontal arrangement of the spines ; 12, same, with all lines obsolete, but both series represented by the vertical and horizontal arrangement of the spines. horizontal lines (figure 10). In other cases the horizontal lines disappear, leaving the spines on the vertical lines (figure 11). Finally, both horizontal and vertical lines be- come obsolete, and then only the spines remain, as shown in figure 12. The indirect production of spines is not always evident, for if the ontogeny or phylogeny of the individual is unknown, 12 STUDIES IN EVOLUTION 13 its direct or indirect development cannot be determined. An excellent example of indirect, or regressive, growth of spines is afforded in the common Barberry {Berberis vulgaris), on the summer shoots of which are shown most of the gradations "between the ordinary leaves, with sharp bristly teeth, and leaves which are reduced to a branching spine or thorn. The fact that the spines of the Barberry produce a leaf-bud in their axil also proves them to be leaves" 24 (figure 13). It should be noted that the process of spine development illustrated in Spondylus (figure M KY* 14) is directly opposed to that of the Barberry. In the former the initial growth is smooth, then faint, concentric, and radiating lines appear, which gradually grow stronger, developing more or less regular inequalities ; and by the excessive growth of these variations spines are formed. In FIGURE is. Summer shoot of the Barberry there are at first Barberry ; showing the gradations no rmal leaves, which are followed between leaves and spines. The , , , , , arrow indicates the direction of b J others more and more toothed growth. (After Gray.) and bristly, until the leaf is rep- imperialis; showing the series of while finally Spines Only are spines. The arrow indicates the f orme d. The Spondylus repre- direction of growth. . . . sents a progressive increase in growth to produce the spines, while the Barberry exhibits a progressive decrease of growth, or an "ebbing vitality," as it has been termed by Geddes. 20 The spines are the final results of both the direct and indirect modes of production; the direct, through a process of building on new tissue, and the indirect, through a process of dwindling away to all but the axial elements. These differences are graphically expressed in figures 13 and 14. ORIGIN AND SIGNIFICANCE OF SPINES 13 Attention should be called to the four kinds of spine production in different organisms. (1) In the Radiolaria, Echinoidea, the Giraffe, Cattle, and the Rhinoceros, the spines or horns are persistent, and grow by additions to the original structure. The new tissue may be superficial, sub- terficial, interstitial, or formed by synchronous resorption and growth. (2) In the Crustacea and Articulata generally, and in the Deer, Elk, etc., the spines are moulted, or shed, periodically. In their various stages, these types (1 and 2) 15 FIGURE 15. Example of spine growth by simple increscence. Horn (left) and horn-core (right) of Ox. (After Owen.) FIGURE 16. Stages of spine growth by successive replacement. Antler series of Red Deer, at ages of 1, 2, 3, etc., years. (After Owen.) FIGURE 17. Stages of spine growth by serial repetition. Profile of a series of spines on one of the primary radii of Spondylus imperialis. FIGURE 18. Stages of spine growth by decrescence. Transformation of leaves into spines in Berberis vulgaris. (After Gray.) can be studied only by means of separate specimens con- secutive in age, or by observing the metamorphoses in one individual. (3) In the shells of Brachiopoda and Mollusca, the stages of growth of the individual are generally retained throughout life, and the successive development of spines may be studied, therefore, in a single example. (4) Spines produced by suppression, as in the Barberry, express their origin through a series of gradations between separate parts ; while in others suppression is brought about by the loss of structures. 14 STUDIES IN EVOLUTION The first type mentioned develops horns or spines by simple increscence (figure 15); for example, the Ox: the second, by successive replacement (figure 16); as in the Deer: the third, by serial repetition (figure 17); for example, Spondylus : the fourth, by decrescence (figure 18); for example, the Barberry. Localized Stages of Growth. By the multiplication of sur- face ornaments through the process of interpolation, many Mollusca present stages of spine development in two direc- tions. (1) The normal series is represented by the succession of spines along a single sector of growth. For instance, in the radial plications of a Spondylus or Lima, the earliest and primitive spines are found near the beak, while those on the ventral border of an adult specimen are the latest and most highly developed (figure 30). These successive stages, there- fore, are in the direction of growth, and may be called longi- tudinal. (2) By the radial divergence of the ribs or plications and the interpolation of additional ones at various intervals, as many transverse compound series of spines finally appear along the periphery as there are primary radii. Hence, in a given case, there may be two radii continuing to the beak, then by interpolation there are successively 5, 11, 23, etc., radii, the highest number being found at the periphery (figures 19, 20). Moreover, by taking the distal spines on these 23 rows, there result the same stages of spine develop- ment as shown in the longitudinal series along any primitive plication (figure 20). A pelecypod shell like Spondylus is here used to illustrate this process, but the application may also be made to the Brachiopoda as well as to the conical non-coiled Gastropoda. In a coiled form like a cephalopod or an ordinary gastropod, the longitudinal lines would follow the whorls spirally, and the transverse lines would corre- spond to the lines or increments of growth of the shell. Species in which the radii are all introduced at an early stage of growth (many species of Cardium, Pecten, Lima) or in which the radii multiply by regular dichotomy would ORIGIN AND SIGNIFICANCE OF SPINES 15 show, of course, only the longitudinal series, for at the margin of the shell the radii would be of the same size and age, and the spines uniform. The foregoing example illustrates an important principle 19 20 44 40* ililiiifi i FIGURE 19. Sector; showing in diagram the multiplication of radiating lines by interpolation. The two primary radii (1,1) are the only ones continuing through the whole four zones. The first zone lias 2 radii ; the second, 5 ; the third, 1 1 ; and the fourth, 23. FIGURE 20. Profiles of the spines produced on the various radii at the four zones ; as indicated in the preceding figure. A, the spines on the two primary radii of the first zone ; B, the spines on the second zone, showing the growth of those on the two primary radii (1, 1), and the small spines on the newly interpo- lated radii (2, 2, etc.) ; C, the spines on the radii in the third zone ; D, the spines at the bottom of the fourth zone. The two large compound spines are on the two primary radii. Their development may be traced by following them through A, B, C, to D. The next three longest spines (2, 2, 2) are tricuspid, and represent the stage of spine development attained by the spines on the radii which were interpolated on the second zone. The next six smaller spines (3, 3, 3, etc.) are on radii which were introduced on the third zone. The twelve small spines (4, 4, 4, etc.) are on the radii introduced on the fourth zone. Thus there are four stages of spine growth shown on the lower margin of the fourth zone, and these corre- spond to the four stages exhibited by the series of spines on one of the primary radii running through the four zones. of ontogeny ; namely, that in organisms which repeat various parts during their growth, these parts will develop or pass through a series of stages corresponding to the initial and subsequent stages of the parts repeated. In this way struc- 16 STUDIES IN EVOLUTION tures appearing late in the ontogeny of the individual will present primitive infantile and adolescent characters. Further development, if such takes place, will pass through a progressive series of ontogenetic changes, and if the stages of growth are by serial repetition and thus are retained in the part, it will be found that such stages can be correlated with those appearing early in the life or history of the indi- vidual. Therefore, in studies of this kind, it is possible to take a structure appearing at maturity, and from it deduce or predicate as to what were some of the early characteristics of the whole individual. This principle is termed localized stages of growth by Jackson, 37 and was first noticed by him in some investigations on Echinodermata. Compound Spines. A simple, sharp, conical process expresses only the primi- tive type of spine. In plants and animals it is the most FIGURE 21. Simple spine. FIGURE 22. Spine, with lateral spinules. FIGURE 23. Spine, with forked apex and lateral spinulose spinules. common form found, and is the first stage of spine differen- tiation. From this type the myriad forms of spines known in the organic world are produced by almost insensible gradations. It is needless to attempt a detailed description of this infinite variety; but, as a single illustration, some of the leading forms of spine differentiation among the Radio- laria are here shown (Plate I). These figures are taken from Haeckel's "Report on the Radiolaria," 26 and generally represent enlargements of from 100 to 400 diameters. Prob- ably no other class of organisms presents greater variety, and ORIGIN AND SIGNIFICANCE OF SPINES 17 many of the forms are repeated again and again, not only in various species of this group, but elsewhere both in the animal and vegetable kingdoms. Whenever the development of a compound spine can be studied, it shows a gradual progress from the simple to the complex (figures 21-23). The antlers of the Red Deer (Cervus elaphus) furnish a familiar example. Fawns of the first year have antlers with only a single prong, a short front tine being added the second year; then "year by year as they are renewed they acquire a greater and still greater number of tines and branches, till they finally attain the complete stage, when their owner is termed a ' royal hart' " 44 (figure 16). Although somewhat conventionalized, the pri- mary series of spines on the Spondylus shown in figure 20 exhibits the passage from simple to compound forms. An inspection of many species of Murex will show the stages in series presenting a greater complexity. After spine development has reached its maximum growth and differentiation, evidence of old age may be exhibited in two ways: (a) The spines may be reduced by resorption, decay, or abrasion, and finally become obsolescent; or what is of greater import (5), they may gradually cease to be developed, as is especially shown in organisms in which spine growth is by serial repetition. Thus, in Spondylus calcifer, a young individual measuring about two inches across has marginal spines fully an inch in length. Even longer spines are found when the shell reaches a width of four inches. On attaining a maximum diameter of about six inches, spine growth gradually ceases, and the margin of the valves is entire and nearly smooth. At this stage shell secretion is confined to excessive thickening of the valves. These senile stages of spine growth will receive further consideration under the discussion of ontogeny and phylogeny of spinous species. Application of Law of Morphogenesis. The manner in which spines arise from plane surfaces, or from the growth 2 18 STUDIES IN EVOLUTION or modification of superficial structures, and also through the decadence of organs, has now been noticed. The spine may thus be taken as a unit for comparison, and its various stages of growth, which were shown to have a definite sequence, may be used in correlation to determine relatively the degree of spine specialization attained by any organism. Further- more, enough data have been already given to lead to the suspicion that spines may represent the limits of ornamental or superficial differentiation or variation. At this point in the discussion this statement must be considered as more suggestive than conclusive. The proof of its reality will be more clearly shown later on. Ontogeny of a Spinose Individual. With few exceptions the embryonic and larval stages of all organisms are devoid of specialized surface features. In other words they are without ornament and without weapons. The exceptions to this rule seem to be readily explained under the principles of larval adaptations and accelerated development. Cases of the latter kind, therefore, can hardly be considered as exceptions, since they represent, not real larval features, but former adult characters which have been pushed back or which develop earlier so as to appear even- tually in the larval or later embryonic stages. In the very earliest stages of embryonic development, the truth of the first statement becomes obvious, and accordingly the pro- tembryonic, mesembryonic, metembryonic, neoembryonic, and typembryonic stages are without surface ornaments or spines. Among Mollusca, the protoconch, periconch, and prodis- soconch, or the early larval shells, are smooth and without ornament. Even the prodissoconch of very highly spinose species, as in Spondylus, is as smooth as that of the plainest species of Ostrea, Anomia, Avicula^ etc. Likewise, the proto- conch of the most specialized or most retrograde cephalopod is perfectly plain. In the nepionic stages the spiny Murex is without spines. In the Brachiopoda the protegulum, or early larval shell, is always without sculpture; while the ORIGIN AND SIGNIFICANCE OF SPINES 19 nauplius of Crustacea and the protaspis of Trilobita are generally spineless. The young of horned vertebrates are almost universally hornless, the Giraffe being the only mam- mal born with horns. The very young seedlings of plants are likewise spineless. In insects the embryonic stages generally have simple cuticles, but in the larval stages of this class and the Crustacea, a great variety of spines and ornamental characters is developed. Altogether, it may be asserted that spines do not appear during the embryonic stages of animals and plants, and that their initial develop- ment is commonly post-larval. Examples illustrating the ontogeny of a spinose form could be multiplied indefinitely, and taken from nearly every class of organisms. In all cases practically the same sequence of events relating to the development of spines would be found. The organism would first be smooth, without sculpture or ornament, like the young of other organisms. At some stage of the ontogeny the beginnings of spines would appear, and develop first into simple, and later, according to the stage of differentiation attained, into compound spines. This pro- gression would finally reach the maximum, spine growth would cease, and the surface of the organism would inversely revert to an early and more primitive type without spines. Normally these changes would represent the infantile, ado- lescent, mature, and early and late senile periods of the life of the organism. In some cases, however, the stages of spine growth, or acanthogeny, do not agree with the ontogeny of the entire individual in respect to time, and here acceleration and the phylogeny of the species will be found to offer the proper explanation of the divergence. As simple examples of the ontogeny of spiniferous species, the Mollusca afford especial advantages, owing to the fact already noticed, that the stages of development are commonly preserved in a single individual. In figure 24 the larval shell, or prodissoconch, of Pelecypoda, or bivalve shells, is represented, and shows the usual type throughout a large portion of the class. The succeeding shell growth of the 20 STUDIES IN EVOLUTION dissoconch is at first generally smooth, save for the fine con- centric lines of growth (figure 25). In ornamented or spinose species, however, irregularities in the growth lines soon appear (figures 26, 27), and these shortly assume the characteristic surface sculpture of the normal adult. Thus the prodisso- conch of Avicula sterna is represented at >, figure 25, and is followed by regular concentric growth during the nepionic 24 25 26 FIGURE 24. Prodissoconch of Ostrea virginiana. X 43. FIGURE 25. Each stage of Avicula sterna; p, prodissoconch. X 19. FIGURE 26. Young Avicula sterna ; showing the beginning of spine growth. X 3. FIGURE 27. Young Saxicava arctica. X 19. FIGURE 28. Young Anomia aculeata; prodissoconch succeeded by early smooth and later spinous dissoconch growth. X 30. (Figures 24-28 after Jackson.) stages. In figure 26 the spiny characters of early adoles- cence are added to the previous stages, and in later stages the spines become more and more emphatic. In Spondylus the prodissoconch is the same simple form, and is succeeded by a nearly smooth Pecten-like stage, dur- ing which the animal was free (figure 29). After fixation the growth is very irregular and ostrseiform for a time, until the shell rises above the object of support, when all the most ORIGIN AND SIGNIFICANCE OF SPINES 21 characteristic features of surface ornamentation become fully developed (figure 30). As the shells approach maximum growth, the spines gradually become shorter, and in old age none are developed, even those of early growth being removed by the action of boring animals and by solution (figure 31). It seems unnecessary to increase the number of examples showing the ontogeny of spinose individuals. The Deer and 29 31 FIGURE 29. Young Spondylus princeps. Eight valve ; showing pecteniform stage succeeded by ostraeiform growth. Taken from apex of adult specimen ; presented by R. T. Jackson. X 3. FIGURE 30. Side view of Spondylus calcifer, about one-third grown; show- ing the characteristic spinous growth. |. FIGURE 31. Side view of Spondylus calcifer ; showing the greatly thickened right valve and the entire absence of spines over the whole shell. \. the Ox may be again cited in this connection. Both are born without horns, but during adolescence the antlers of the Deer become longer and more complicated with each renewal, while the horns of the Ox are longer and more twisted. In old age, when the Deer has passed his prime, the antlers are more obtuse, and exhibit a tendency toward decline and obliteration. Suppression of the antlers is accom- plished by the removal of the cause of antler growth and specialization, so that the unsexing of the male results in 22 STUDIES IN EVOLUTION small antlers, which are seldom branched, and become thick- ened by irregular deposits of bone (Owen 53 ). Spines grow during the adolescence of the Horseshoe Crab, Limulus polyphemus, yet in old age they are obsolescent, being repre- sented by rounded nodes. As examples illustrating the accelerated development of spines in widely separated classes, the Giraffe among mam- mals and Acidaspis among Arthropoda may be selected. The Giraffe represents the continuance of a very primitive type of horn ; namely, one covered with a hairy skin. They are never shed, and are common to both sexes. Out of this type all others found among the Mammalia have probably been devel- oped. The point of interest here is that the young Giraffe is born with horns, and as these could serve no prenatal purpose, it must be concluded that the action of accelerated heredity has pushed the development of these organs so far forward as to cause them to appear during foetal growth. The next illustration of acceleration is taken from the Trilo- bita. Acidaspis is one of the most highly specialized and ornate genera. Although the larval forms of other genera are commonly without ornament, yet in the present genus the protaspis, or phylembryonic, stage partakes of this specializa- tion in so far as to develop minute spines, which later become larger, more differentiated, and form a conspicuous feature of the adult. Other characters have been likewise shown to appear at an earlier period than in other genera, and the earlier inheritance of spines must be explained in the same manner. 8 The facts, as stated, seem to warrant the conclusion, that in spinose organisms the very young are almost universally with- out spines. Acceleration may occasionally push their devel- opment into the embryonic and larval stages, but ordinarily they are not so subject to the action of this law as are some of the physiological and other structural characters. This will be explained as in part due to the lack of general plasticity, and because differentiated spine growth is the progressive limit of variation. Therefore there are no subsequent characters to displace them and crowd them forward in the ontogeny. ORIGIN AND SIGNIFICANCE OF SPINES 23 Phylogeny of Spinous Forms. To interpret phylogeny in terms of ontogeny, according to the law of morphogenesis, or recapitulation, is perhaps easier than to trace a genetic sequence through a series of forms having a considerable geologic range. Taking the ontogenies of the animals already noticed, there is for the Pelecypoda the prodissoconch, which is correlated by Jackson 36 with Nucula, and a Lower Silurian nuculoid radicle is assumed for the Aviculidse and allied forms. The first dissoconch growth pro- duces a shell resembling Rhombopteria, a Lower and Upper Silurian type, and this is taken to represent the second stage in the phylogeny of Avicula (figure 25), Anomia, Spondylus , etc. Continuing the development of Spondylus, it is found by Jackson that it passes successively through stages which may be correlated with Pterinopecten (Devonian), Avion- lopecten (Devonian), Pecten (Carboniferous ?), and Hinnites (Trias), while finally it assumes true spondyliform characters. These correlations agree with the geologic sequence of the genera, and are believed to indicate phylogenetic relationships. It may be further remarked that the early species of Spondyli are more truly pecteniform and hinnitiform than the later ones. The genus ranges from the Trias to the present. Zittel 73 remarks that " the oldest species are small, thin-shelled, and seldom much ornamented." Even in the Cretaceous, the majority of species are not far removed from Pecten and Hinnites. During the Tertiary the irregular, ostrseiform, squamous, concentric, and spinous growth becomes more manifest, and at present most of the species show a great development and differentiation of the spines. Thus, while Spondylus is normally considered as a spinose genus and the species are familiarly known as Spiny Oysters, yet, as it is traced back in geological history, the forms become less and less spinose, and their affinities and appearances are more and more in accord with non-spinose genera, until finally the prototype is a smooth, simple, delicate, unornamented shell. 24 STUDIES IN EVOLUTION The simple antlers of the young Deer and Elk correspond in type with those of the adults of the Middle Tertiary Deer (Lydekker 44 ), and it may be therefore assumed that the great number of branches and tines is a modern development. Further back in the Tertiary the ancestors of the Deer were without antlers, thus representing in phylogeny the new-born Deer of the living type. These correlations are made from comparisons of chronogenesis, or development in time, and ontogenesis, or development in the individual. An example of a different kind will now be given to show more clearly a genetic sequence in forms. Among the Brachiopoda, Atrypa hystrix represents one of the terminal members or species of a line of varietal and specific differ- entiation, extending through the Silurian and Devonian. The type commonly known as Atrypa reticularis appears to have had its inception during the Ordovician ; yet in the Silurian it is found as a conspicuous and fully developed form. Here, also, it has quite a wide range of variation, but there seems to be an insensible gradation between the extremes, which therefore cannot be considered as definite permanent varie- ties. There are, however, associated forms that have received distinctive specific names, which do not shade into each other. During the early and middle Devonian certain of these varia- tions in the main stock of A. reticularis became more fixed, and at the time of the Hamilton sediments in New York, there are two forms known as A. reticularis and A. aspera, which apparently do not pass into each other. As time went on, these two types became more specialized and the diver- gence correspondingly increased, until in the Upper Devonian, in the Chemung sediments, there is a large many-plicated A. reticularis, as well as a form with very few plications and long marginal spines, A. hystrix. Hall and Clarke 31 thus summarize the stages leading to the formation of the spinose forms: "In the variant of Atrypa reticularis, occurring in the Niagara fauna at Waldron, Indiana, the free concentric lamellae frequently show a tendency to fold inward at the summit of the principal plications. The infolded edges fail ORIGIN AND SIGNIFICANCE OF SPINES 25 to unite, and this tendency to the formation of tubules is apparently carried no further at this period. More extreme results were attained by the Atrypa aspera of the Hamilton shales, or possibly by its migrated ancestor, during the period of time represented by the deposition of the Lower Helder- berg, Oriskany, and Upper Helderberg sediments. At all events, the Atrypa spinosa of the Hamilton shales is but an A. aspera with the lamellae enfolded into tubular spines. Intermediate stages connecting these different phases are not present in this fauna. . . . This spinose form is continued into the Chemung faunas (A. hystrix), with some modifi- cation of expression, the spines being few and long, and the plication of the surface very coarse and quite simple; the shell in its decline thus representing a decided return to the primitive type of structure." H. S. Williams 72 has classi- fied the variations in the stock of A. reticularis as to whether differentiation in the number of plications is increased or retarded, and concludes that the extremes are most strongly expressed at the close of the life-period of the race. The numerously plicated type represents the accelerated phase of the multiplication of radii, while A. hystrix, with its few and coarse radii, represents the retardation or suppression of this tendency. The only great group of animals receiving its name from its characteristically spinose surface is the Echinodermata, or the spiny-skinned animals; yet it is extremely doubtful whether this name would have been used had the first studies of the group been based upon the Paleozoic representatives, especially the pre-Devonian species. The early Sea-lilies (Crinoidea), Cystideans (Cystoidea), Blastoids (Blastoidea), and Star-fishes (Asteroidea) had smooth or nearly smooth integuments. In its early genera, even the most typically spiny class of the whole sub-kingdom, the Echinoidea (Sea- urchins), had very minute and insignificant spines. It is only in the late Devonian and in the Carboniferous that truly spiny forms of Crinoids, Star-fishes, and Sea-urchins are found. 26 STUDIES IN EVOLUTION Of equal significance is the fact that the Echinodermata together with the plants represent the most primitive type of structure, one in which there is a more or less circular arrangement of the parts or organs. The Echinodermata are the highest development in this line of growth among animals. They culminated in past geological ages, and from them no direct line of descent can be traced (Bailey 2 and Cope 11 ). The conclusion from the study of the phylogenies of spinose forms is parallel to the one drawn from the ontogenies ; namely, that the ancestors of spinose as well as non-spinose organisms were simple and inornate. CATEGORIES OF ORIGIN As previously shown, spines are formed either by growth or by suppression, and therefore the processes determining their production are either constructive through concrescence or destructive through decrescence. Each of these is in turn determined by forces from without the organism (extrinsic) or by forces from within (intrinsic). In this connection it is of no especial moment whether or not the intrinsic forces are primary or are an immediate or subsequent reflex from the extrinsic. The main thing is the direction of the dom- inant force, whether centripetal or centrifugal. If in some cases it can be shown that spine development has been accomplished by intrinsic forces in the organism, then this development may be brought about independently of the environment and possibly at variance with it. Also, if in other cases the extrinsic forces or the influences of the environment have caused spine growth, it may in some instances illustrate the formation and transmission of an acquired character, or at least the operation of organic selection. The point has now been reached where it is impracticable to make a rigid classification of the direct factors or an exact determination of primary and secondary causes. It ORIGIN AND SIGNIFICANCE OF SPINES 27 was remarked at the beginning of this paper, that single causes were not sufficient in every case to account for spine growth, and while it is comparatively easy to formulate abstract expressions or terms covering all possible cases, it will be found difficult to construe properly certain factors to fit into any particular conception. In illustration of this, the foregoing statements may be taken. Thus spines are formed by the only means possible, either by growth of new tissue or by decrease in old. Again, the forces must act from the interior or from the exterior ; in other words, they must be intrinsic or extrinsic. But in some specific instance, while considering food, forces of nutrition, external or in- ternal demands, reactions, etc., a question may arise as to the proper disposition to make of a spine developing primarily by external stimuli and becoming a defence and secondarily a weapon ; yet which by differentiation in time loses some of its protective and offensive qualities, and by selection may be confined to one sex. Growth and decline are underlain by the processes taking place in individual cells as well as in aggregates of cells, for spine growth must be considered in unicellular as well as multicellular organisms. Ryder 61 has very philosophically discussed the correlations of volumes and surfaces of organisms, and has reached the conclusion that " the physiological function of a cell is also a function of its figure, i. e., of its morphological character ; that is to say, cells tend to elongate in the direction of the exercise of their function." Out of this may be deduced the correlative conclusion that aggregates of cells having a like function also tend to elongate in the direction of the exercise of this function ; and, further, it may be asserted that parts or portions of cells will act in the same manner. A familiar illustration of these principles as applied to a single cell may be taken from the rhizopod Amoeba proteus. When disturbed by incident forces in all directions, it assumes a globular form. Under continuous motion of its own, it is elongated in the axis of motion, its larger pseudopodia being 28 STUDIES IN EVOLUTION thrust out in more or less the same direction. The presence of a favorable exciting cause, like a particle of food, produces extension of the protoplasm to envelop it. Furthermore, as is well known, continuous extra-pressure on any part of an organism produces atrophy and absorption, and intermittent or occasional pressure causes hypertrophy and growth. That the pressure should be intermittent seems a necessary condition for hypertrophy, in order that the parts affected may have normal intervals allowing the active exercise of nutrition. 55 This may be regarded as a parallel statement of the law of disuse and use ; the former causing organs or parts to dwindle away and lose their function, and the latter producing increased nutrition and growth. This ratio of exchange between nutrition and waste is on the side of full or excessive cell-nutrition, producing growth in the parts affected, while deficiency of nutrition produces decline or suppression. If the successive increment constitut- ing growth is along definite progressive lines towards higher structures, and the decrement affects the decline of useless parts or permits of the replacement of a lower by a higher structure, then the sum of the changes is progressive evolution.* Growth, as stated, seems to require normal intervals for the proper exercise of nutrition, which involves an inter- mittence of the exciting or stimulating forces. Rhythm has been shown by Spencer 66 to be a necessary characteristic of all motion, and therefore in considering either the intrinsic or extrinsic forces acting on the structures of an organism, they must be rhythmic or intermittent. In the environment the most apparent changes are those of light and darkness, heat and cold, moisture and dryness, and variations in amount of oxygen, all of which affect an organism directly, and also through the accompanying variations in the character and amount of the food supply, the number of enemies, etc. These and most of the mechanical forces of the environment are therefore intermittent, and their resultant must have a * This is very near Cope's idea of progressive evolution. ORIGIN AND SIGNIFICANCE OF SPINES 29 definite tendency, so that the effects are not with each change successively positive and negative to the same degree ; that is, the same structures or adjustments are not alternately made and unmade. It is generally recognized that there is a necessity for a force or energy in living organisms, which is not the imme- diate and direct result of external agencies, but upon which these fall and produce reactions. It is considered as a phase or kind of vital force directing growth, and therefore a growth force, or the bathmic force of Cope. 10 The internal energy of growth, involving the capacity or effort of respond- ing to external stimuli, is termed entergogenic energy by Hyatt. 34 Without this power an organism would be unable to move or respond to external stimuli. The effect of the action of this kind of energy must be the resultant between "the structures already existent in the organism and the external forces themselves." 34 Since the growth force is within the organism, or inborn,, it is one of the principal characters transmitted through heredity, and if it is in excess of the external forces, the modifications will be principally congenital or phylogenic. If, on the other hand, the external forces predominate, the modifications will be principally adap- tive, or ontogenic. In each case the resultant is the actual visible effect of the two. If both are toward the establish- ment of similar structures, their effect will be the sum of the two; but if they are opposed to each other, the effect will be their resultant, the nature of which, as seen above, will depend upon their relative power. These conclusions can be correlated directly with the developmental variations occurring in the life history of any great group of organisms. Any one who has studied the chronological development or the phylogeny of a class of forms cannot fail to have been impressed with the fact that all types of life are physiologically more plastic or subject to greater changes near their point of origin. That is, the maximum of generic, family, and ordinal differentiation is 30 STUDIES IN EVOLUTION found at an early period, while the greatest specific differen- tiation occurs at a later period. This shows that the results of variation at first affect the physiological and internal structures, and that later the changes are mainly physical and peripheral. One explanation of this would be that the forces of the environment are at first freely transmitted and produce internal modifications, and that later these characters become stable, making the effects of the external stimuli apparent in the superficial differentiation of the organisms. In any event the modifications in function and structure are followed by modifications in surface, showing that the more important physiological and structural variations are the first to be subjected to heredity and natural selection, which tend to fix or hold them in check. Features of less functional importance, as peripheral characters, are the last to be controlled, and therefore present the greatest diversity, while in this diversity spinosity is the limit of progress. In order to be hereditable, the modifications through the environ- ment must have induced correlative internal adjustments and changed forces which can be transmitted to offspring, and they in turn reproduce the specific modifications. For the purpose of illustrating these statements, the evolu- tion of the Brachiopoda and Trilobita will be taken. The Brachiopoda are divided into four orders, all of which appear in the Lower Cambrian and continue to the present time. Schuchert 64 states that "of the 49 families and subfamilies constituting the class, 43 became differentiated in the Pale- ozoic, and of these 30 disappeared with it ;" also, "of the 327 genera now in use, 227 had their origin in Paleozoic seas, or nearly 70 per cent of the entire class." Throughout the Cambrian, "differentiation was mainly of family importance." "Differentiation is most rapid near the base of the older systems, and diminishes the force from the older to the younger geologic divisions." The most rapid increase was in the Ordovician, the culmination was in the Devonian, and the rapid decline came with the Carboniferous. About six ORIGIN AND SIGNIFICANCE OF SPINES 31 thousand species are known, and of these probably not more than one hundred and fifty are living. Similar data are derived from the Trilobita. This group is found all through the Paleozoic, at the close of which it became extinct. Two of the three orders are found in the Lower Cambrian. The remaining order appeared just after the close of the Cambrian in the early Ordovician, yet through the whole of the remaining sediments not a single new ordinal type was developed. When applied to a single order, the same truth comes out. The order Proparia is one whose entire history can be traced, extending from the Ordovician through the Silurian and Devonian. All the families appear in the Ordovician ; in fact not a single family type in this or the other orders was produced during the whole Silurian, Devonian, and Carboniferous. 6 As the classes, orders, and families are based upon the physiological and important functional structural characters or differences, it is evident that at or near the beginnings of their life history is found the demonstration of the domina- tion of phylogeiiic over ontogenic characters. Conditions or Forces affecting Growth. Since spines are purely organic structures, their production must follow general laws of organic change. The forces considered as of c most consequence are two : (1) the external stimuli from the environment, and (2) the energy of growth force. These, with their opposites (1 a) the restraint of the environment, and (2 a) the deficiency of growth force, are believed to include the chief active and passive causes, not only of spine production, but of growth and decline in general. Correlat- ing these four causes with their constructive and destructive agencies, together with their extrinsic and intrinsic modes of action, as previously explained, there result (A) the external stimuli of the environment as an extrinsic cause of concres- cence; (B) energy of growth force as an intrinsic cause of concrescence; (C) external restraint as an extrinsic cause of decrescence ; and (D) deficiency of energy of growth force as an intrinsic cause of decrescence. The remaining vital 32 STUDIES IN EVOLUTION forces (nerve force, or neurism, and thought force, or phren- ism) are not primary, and, although doubtless affecting growth in higher organisms, cannot be original causes appli- cable to all forms of life, both plant and animal. In tabular form, the divisions and relationships of the factors of spine genesis may be expressed as follows: A r ( extrinsically \ from external / (ceutripetally) t stimuli, 'constructive agencies J (concrescence) acting "j B I ( intrinsically / from growth * ( (centrifugally) J force. Spines originate by^ C ( extrinsically ) from external , 1 (centripetafly) i restraint, destructive agencies I (decrescence) acting [ D intrinsically ) from deficiency (centrifugally) ) of growth force. Under the last four divisions (A-D) it is proposed to discuss the origin of spines, and from the observations made, to derive certain conclusions regarding the significance of the spinose condition. A. External Stimuli. Under external stimuli are included all the forces of the environment (chemical, physical, organic, and inorganic) which, through their impact or influence on an organism, produce a consonant favorable change or disturbance. In general, it will be seen that the number of impressions and their power will depend largely upon the position and char- acter of the surface upon which they impinge. The more exposed the position, the greater will be their strength and number, and if these stimuli or impressions are intermittent, and not so violent as to produce waste and rupture, growth will ensue. Under ordinary conditions, exposed parts will naturally be the first to receive sufficient stimulus to produce growth, and there will be normally a direct correlation be- tween growth and stimulus. In a simple diagrammatic form, ORIGIN AND SIGNIFICANCE OF SPINES 33 this would be expressed by a series of lines, the first repre- senting a plane surface. Then, owing to the impossibility of maintaining a uniformly intermittent stimulus or a uniform response, some point or spot on this surface would grow in excess of the others. This difference would be augmented by the more favorable position of the spot to receive stimuli, further growth would take place, the growth force decreasing with the increase of distance, and the final action of these forces, stimulus and growth, would be to produce a pointed elevation. Such structures or outgrowths, especially when made of hard rigid tissue, would be termed spines under the general definition. The spine may be viewed as an attached organism, and its conical habit of growth would then con- form to the law of radial symmetry, as determined by the physiological reaction from equal radial exposure to the environment. That all the irregularities of contour in all organisms have not developed into pointed processes or spines is not, therefore, the fault of the simple reciprocity between growth and external stimuli. This kind of develop- ment, however, requires a direct and immediate responsive external growth to the exciting force, which from various causes is frequently absent. Obviously, stimuli which result simply in motion or equivalent internal adjustments can have no effect toward spine production, so that only the results of such stimuli as bring about some accompaniment of super- ficial growth will be considered. With the exception of perfectly spherical, freely moving forms, all organisms have certain parts which are more exposed to the forces of the environment than others, and from the principles already enunciated, such exposed parts under normal conditions will grow. This growth in the direction of function and stimulus, when acted upon by the hereditary functional and structural requirements of the organism, serves to produce the various external organs and appendages. But when the surface upon which the stimuli fall is not thus predetermined by heredity to grow into a certain organ or functional part, there results a normal 3 34 STUDIES IN EVOLUTION responsive action between growth and stimulus, which, as already seen, tends to produce a conical or spiniform growth. Under ordinary favorable conditions, simple external stimuli acting blindly through no agencies of selection would develop spines on all the most exposed parts, and tend to differentiate ornamental features. This has been the case with many organisms and colonial aggregates possessing no power of selection or not acted upon by any forces of determination, conscious or unconscious. In such cases spines may or may not serve for protection, and their function, if any, can be only determined separately for each case. If, however, the added function of offence is included, it is manifest that the spines must be located in special positions adapted to use for offensive purposes, as on the tails of some animals, and not necessarily over vulnerable parts. Here the selective agency of special adaptation is shown. Again, if while there is agreement in other essential characters, spines or horns are confined to either sex, it is evidently a case of sexual selec- tion. Further, if they develop in harmony with the environ- ment, or in a manner parallel to similar features of other organisms, it is through the operation of physical selection. Altogether, under the general forces of external stimuli, there are five aspects in which to consider the production and growth of spines ; namely, A. From External Stimuli. A 1. In response to stimuli from the environment acting on the most exposed parts. A 2. As extreme results of progressive differentiation of ornaments. A3. Secondarily as a means of defence and offence. A 4. Secondarily from sexual selection. A 5. Secondarily from mimetic influences. B. G-rowtJi Force. In unicellular organisms growth force, or bathmetic energy, must reside wholly in the germ cell, and therefore is con- cerned with reproduction as well as with cell differentiation. ORIGIN AND SIGNIFICANCE OF SPINES 35 In multicellular organisms the growth force is in both germ and soma cells, and its relative strength seems to depend upon its power to reproduce lost parts, often including germ cells as well as soma cells. In many of the lower classes the growth force is able to complete a structure or lost part without the stimulus of use, which in higher animals often seems to be part of the necessary requirements for growth. Growth itself is the repetition of cells under nutrition and stimulus, and the latter may be hereditary or extra-indi- vidual. It is now recognized that since the division of a cell makes two unlike cells, each unlike the parent, such repeti- tion will produce structures which present some degree of difference. The variation is therefore a necessary quality of growth, and its degree will change in response to the differentiation of the forces affecting growth. When spines which have arisen from intrinsic growth force only are sought, it is apparent that they cannot be distin- guished from those arising from external stimuli acting on and directing the growth force, unless in some instances they are found to be developed independently or even at variance with the environment. Because spines are some- times useful to the organism, it is impossible to believe that they have originated from that cause, since their existence in some form must precede the capacity of making them useful. After they began to develop by either intrinsic or extrinsic forces, their being found useful would simply tend to their conservation and further development. Variation which is not restricted by natural selection or a long line of hereditary tendencies is known as free variation. It is best exhibited in a stock which occupies for a consider- able time a region favorable in respect to food, climate, and absence of dominating natural enemies. This relation has been called the period of " Zoic maxima " by Gratacap, 23 and has been further discussed by the same author, under the aspect of numerical intensity. 22 The most rapid rise of a stock is considered to be consequent to a favorable environ- ment and high vitality. 36 STUDIES IN EVOLUTION In illustration of these points, the AchatinellsB of the Sand- wich Islands afford a good example. The great number of species on these islands has probably been evolved since Tertiary times, and the process of specific delimitation is apparently still going on, for species are now to be found which did not exist fifty years ago (Verrill); also, a few species formerly common are now obsolescent or extinct. According to Hyatt, they all can be deducible from a single species which has differentiated in time through divergence, dispersion, and colonial isolation. In early times birds may have fed upon them, but the complete or partial extinction of the former by man has resulted in complete immunity for the arboreal Achatinellse, and it is now common to find several of the most highly colored varieties feeding together on the same leaf. The modern importation of pigs, sheep, and mice on the islands has introduced an enemy to the terrestrial species, the effects of which are already being noticed. In specific differentiation and in individual varia- tion, both Hyatt and Verrill regard the extraordinary develop- ment of this type as characteristic of free variation, under favorable conditions, in a plastic stock which has not yet reached its limits nor become fixed. Among the Crustacea, the remarkable evolution of the genus Grammarus in Lake Baikal, 17 and of Allorchestes in Lake Titicaca, 19 seem to furnish parallel examples. Allor- chestes ranges from Maine to Oregon and southward, through the United States, Mexico, and South America, to the Straits of Magellan. Before Lake Titicaca was explored, but one or two authentic freshwater species were known from both continents; yet from this lake basin alone, Faxon 19 has described seven distinct species, constituting the entire crustacean fauna with the exception of a species of Cypris. Several species are "remarkable among the Orchestidse for their abnormally developed epimeral and tergal spines." These and the species of G-ammarus from Lake Baikal will be referred to again later in this paper. It is simply desired here to indicate that these variations in Achatinella and ORIGIN AND SIGNIFICANCE OF SPINES 37 Allorchestes have arisen from a single parent stock, within a small geographic province. The natural interpretation seems to be (a) that the environment is favorable, as evinced from the great number of individuals; (>) that this has favored and increased the growth force ; and (c) that, finally, the law of multiplication of effects, reproductive divergence, 67 the survival of the unlike, and the conservative forces of natural selection and heredity have directed the growth force, and produced the specific differentiation which is now found. A factor of Evolution, called " Reproductive Divergence " by Vernon, 67 seems to be operative here, since it affords an explanation for a means of differentiation in a single stock under a common environment. As this factor has but recently been discussed, it may well be defined at this time, so as to enable a direct application to be made. Reprod tive divergence assumes that in many species there will b( greater fertility between individuals similar in color, form, or/ &/L size, than between individuals not agreeing in these respects, / ( ^^ and that in subsequent generations the divergence will ./^ become progressively greater in respect to the characteristic y % in question, so that finally the original stock will become^' *-'' separated into distinct varieties, sub-species, or species. When, from any cause, the forces of nutrition are directed toward spine production, and when the direct results are . ./ accomplished in the reciprocal formation of one or more spines, there is often an apparent inductive influence or impulse given to growth toward the further production or repetition of spines. This may result in the formation of compound spines, or a group of spines, or even produce a generally spinous condition. Naturally, spines arising through growth force may be useful for defence and offence, and the selective influences of sex and mimicry may also tend to greater development and elaboration. Furthermore, growth forces reacting on any external structures, as lines, lamella, ribs, nodes, etc., may tend to differentiate such ornaments into spines. Therefore, under the general consideration of spines pro- 38 STUDIES IN EVOLUTION duced through growth force, the following factors are offered for consideration: B. From Growth Force. B 1. Prolonged development under conditions favorable for multiplication. B 2. By repetition. B 3. Progressive differentiation of previous structures. B 4. Secondary development through the selective influ- ences of defence, offence, sex, mimicry, and other external demands. C. External Restraint. Intermittent stimulus, as previously shown, produces growth in the direction of function. When the growth equals the waste, an equilibrium or static condition is reached, and no relative change occurs. The absence of either extrinsic or intrinsic stimulus will not be favorable to growth, and under such conditions an organ or struc- ture may remain undeveloped, or, if already present in the organism, it may waste away and degenerate into a vestigial structure, or even disappear altogether. On the other hand, it is well known that continuous pres- sure not only prevents growth, but in addition resorption takes place, and in this way the whole or a portion of a structure may be removed. These changes have frequently been studied in embryos, as well as in many internal struc- tures, and are also familiar in the enlarged pedicle-openings of many Brachiopoda, caused by pressure of the pedicle, and in the similar opening for the byssal plug of Anomia. Packard 54 gives examples among the Crustacea and Insecta, which are clearly to the point. He says of the Crustacea, "It may here be noted that the results of the hypertrophy and overgrowth of the two consolidated tergites of the second antennal and mandibular segments of the Decapod Crustacea, by which the carapace has been produced, has resulted in a constant pressure on the dorsal arches of the succeeding five ORIGIN AND SIGNIFICANCE OF SPINES 39 cephalic and five thoracic segments, until as a result we have an atrophy of the dorsal arches of as many as ten segments, these being covered by the carapace." The restraint of the environment through unfavorable con- ditions is the antithesis of A, or the influence of constructive external stimuli, and is considered as the extrinsic operation of destructive agencies. It is evident that external unfavor- able conditions will repress growth, with a resultant atrophy of the structures affected. In this way, also, the environment may cause the disuse of an organ, which by consequent sup- pression may dwindle away to a spine, as in the leaves and branches of desert plants, and the spurs of the Python 60 representing the hind limbs. It may likewise repress growth, as in the spines on the lower side of the poriferous coral Michelinia favosa,^ representing aborted attempts at bud- ding, the failure being due to the unfavorable position of the buds for securing food. The restraint of the environment may also act in a mechan- ical manner to produce spines, as will be shown subsequently in some Brachiopoda and Trilobita. Furthermore, spines arising through any phase of external restraint, may second- arily come under the influences of natural selection, and be useful for protection and offence, or conform to other external demands. Under the head of external restraint, therefore, are the following categories: C. From External Restraint. C 1. Restraint of environment causing suppression of structures. C 2. Mechanical restraint. C 3. Disuse. C 4. Secondarily for protection, offence, etc. D. Deficiency of Growth Force. The growth force in organisms may be reduced in several ways, the most general and obvious modes being by an u- t -i (After Leid ) IcLTia jolium y show that similar FIGURE 35. The same ; spines were developed at different showing a single spine on the t f growt h, SO that, in a full- fundus. X 175. (After Leidy.) 6 5 . FIGURE 36. The same; show- grown specimen, there may be two ing two spines, x 175. (After or three pairs of spines along the sides. Others, like Verneuilina spinulosa* and Colivina pygmoea^ develop spines from the points of each chamber. A number of species, also, show a single spine at the apex of the shell ; as Pleurostomella alter- nans, g Bolivina robusta^ Polymorphina sororia, var. c-uspi- data,* etc. In the latter species the ordinary form is rounded or obtusely pointed at the fundus. ORIGIN AND SIGNIFICANCE OF SPINES 45 Some of the Infusoria have terminal spiniform processes, which, by analogy with other forms, have probably developed according to I; as Ceratium tripos? 0. longicorne,* C.fusus. 9 The apertural spines on some of the graptolites are on the most exposed portions of the hydrotheca; as in Monograptus spinigerusf 2 Dicranograptus Nidiolsonif* Retiograptus tenta- culatus, and Graptolithus quadrimucronatus. In many com- pound corals the corallites are polygonal from crowding, and the most exposed portions, the angles of the calices, often bear spines; as Favosites spinigerus^ Callopora exsul, etc. The spines on the septa and costse of corals probably originate by intrinsic forces (B), since they are internal growths not influenced directly by external stimuli. The spines on the ventral sacs of Crinoidea are usually terminal, and in the most exposed situations; as in Scy- talocrinus validus,^ Dorycrinus unicornis, Aulocrinus Agas- sizi^ etc. The anterior and posterior pairs or rows of spines on the loricse of some species of Rotator^a are in the most exposed places; as in Anurcea squamula, Noteus quadricornis, etc. The spinules on the tubes of Spirorbis are usually developed after it rises above the object of support so as to be exposed on all sides; as Spirorbis spinuliferus. 51 The spinules at the corners of the angular cell apertures of many Bryozoa are in the most exposed situations, and probably arise through external stimuli; as in Trematopora echinata,* T. spiculata,^ etc. The large marginal spines of the brachiopod Atrypa hystrix 31 probably owe their excessive development to external stimuli, though the phylogeny of the species shows that the spines first originated through the differentiation of the radiate and concentric ornaments. In many pelecypods the siphonal region receives a great amount of stimulus, and the post umbonal slope is the part most exposed. Along this slope are found many of the spines, and generally the greatest differentiation of ornament. Examples of spines on post-umbonal slopes may be seen in Oallista sublamellosa and young Saxicava arctica (figure 27). 46 STUDIES IN EVOLUTION Such spines represent periodic extensions of the mantle border, and in some cases the stimulus for this growth may come from internal causes. The spines on Unio spinosus and related species are believed by Mr. Charles T. Simpson to assist in anchoring the shell in the sand of swift running streams. In Callista, the young Saxicava, and the Unios mentioned, the spines occur on all individuals and at such an early period as to preclude any special sexual function. In the Gastropoda the periodic extension of the shell over the posterior canal and the spiniform prominences formed on the labrum are situated in exposed places, or where the amount of stimulus is greatest; as in Trophon magellanicus, Strombus pugilis, Fusus coins, Clavatula mitra, Melo dia- dema, etc. The spines on the larvae of geometrid moths are usually on top of the loop, and are explained by Packard 54 as follows: " The humps or horns arise from the most prominent portions of the body, at the point where the body is most exposed to external stimuli." When the origin and function of spines in a great many forms of animals, and especially among the higher classes, are examined, it seems almost impossible to decide whether a spine has been originated and perpetuated by free variation and heredity, or by the general action of external stimuli on the most exposed parts; and in the latter case, whether or not under the selective influences of use. Its origin in either instance may be through external stimuli, but in the latter, it falls under other captions than A A ; or, in other words, the external stimuli excite the growth force at certain points, and the growths so produced may be simply reciprocal without function or they may serve purposes of protection or offence. Thus the dorsal and rostral spines on the zoea of the Decapoda are on the most exposed points, and seem to function as defensive structures. As soon as the legs become well developed or when the animal ceases to swim at the surface and hides among the stones, etc., at the bottom, these spines become reduced and are often succeeded by ORIGIN AND SIGNIFICANCE OF SPINES 47 others. The spines of the adult are also usually efficient for protection, but owing to the change in form of the animal and change of habitat, the most exposed parts are different from those of the larva, and the spines are frequently developed where there were no larval spines; as in Cancer irroratus, Callinectes hastatus, etc. Again, the horned ungu- lates show in their habits of sport, fighting, defence, and procuration of food, that the exposed angles of the top of the skull are subject to the greatest number of stimuli, and there the horns are developed. The connection between external stimuli and growth is here most manifest, for it is impossible to imagine the action of free variation or simple growth force as resulting independently, in the evolution of horned out of hornless species in several sub-orders of mammals, and in every case determining the location of the horns on the prominent angles of the skull, whether on the nasals, ; maxillaries, f rentals, or parietals. It is well known that toads and frogs defend themselves by using the head as a shield, and the cranial angles thus receive the greatest amount of stimulus. " There are natural series of genera measured by the degree of ossification of the superior cranial walls " (Cope 10 ). In the highest genera the head is completely encased, and in some forms the project- ing angles are developed into short horns. The so-called " Horned Toad " (Phrynosoma) has the same habit of defence, and it is believed that this mode of protection or of receiving impacts has given rise to the structure, by stimulating growth at these points. II. As extreme results of progressive differentiation of previous structures. (A 2 , B 3 .) The differentiation of existing ornamental structures into spines has already been noticed in several instances in this article. It was shown that spines often arise by the elonga- tion of nodes and tubercles or similar structures, by rhythmic alternating areas of growth in lamellae and ridges, and by 48 STUDIES IN EVOLUTION the growth of matter at the intersections of lines, lamellae, ridges, etc. Furthermore, it was indicated that this progres- sive differentiation could be produced either (a) by the direct action of external stimuli affecting the amount of nutrition brought to a certain structure, (6) by the stimulus and dis- persion of growth force, or (c) by a combination of the two forces. In this differentiation of the features which are generally called "ornamental," it will also be shown that the spine is the final result of progressive differentiation, and, as ? previously indicated, can be formed out of a variety of other structures. The term " ornamental " is mainly one of human interpretation, and is used simply in apposition to "plain" or " simple ; " for example, a clam cannot be imagined as consciously favoring a particular kind or arrangement of tubercles for ornamental purposes. In a reticulate or cancellate surface formed by the crossing of raised lines, ridges, or lamellae, it is evident that the causes or forces producing such structures will be increased at the points of intersection, and normally the amount of growth will here be greatest. In this way it is possible to account for the very common presence of spines at the intersections of the radiating and concentric lines on many Mollusca and other organisms. A few examples will now be given illustrating the differ- entiation of various structures into spines. The points of intersections of the elements of the lattice in the Radiolaria are where spines are most frequently found ; as in Larnacalpes lentellipsis, Orosphcera Huxleyi, Carposphcera melitomma, etc. 26 In Xiphosphcera pallas, the ridges about the openings or meshes are granular, and the intersections are raised into spines. Many of the discoid shells have their edges differentiated into spines; as Heliodiscus asteriscus, H. cingilluni) H. glyphodon, Sethastylus dentatus, Heliodry- mus dendrocydus, etc. When an edge becomes elevated and defined as a carina, this structure is also often spiniferous ; as in Tripocalpis triserrata and Astropilium elegans. The final differentiation of the radiate arrangement in the Radiolaria ORIGIN AND SIGNIFICANCE OF SPINES 49 results in forms consisting only of a composite spine ; as in the legion Acantharia. In the Foraminifera there are many instances of the gradual differentiation of carinse, ribs, costse, etc., into spines. In Bulimina aculeata 9 the surface nodes and granules become developed into spines. In Textularia carinata 9 and Cristel- laria calcar Q the carinae are spiniferous. The young of Uvigerina aculeata 9 is strongly costate, and later shell growth shows the costse broken up into numerous spines. A re- lated species ( U. asperula 9 ) has the whole test covered with spinules, which are sometimes arranged in lines, showing derivation from costse. In Truncatulina reticulata 9 the carina is made up of confluent spines, often discrete along the edge, and sometimes entirely separated. 37 38 39 FIGURE 37. Cyaihophycus reticulatus. Ordovician. . FIGURE 38. Dictyospongia Conradi. Devonian. \. FIGURE 39. Hydnoceras tuberosum. Devonian. . (Figures 37, 38, 39, after Hall.) To illustrate progressive chronogenetic and ontogenetic differentiation in a family of hexactinellid sponges. The hexactinellid sponges belonging to the family Dictyo- spongidse show some very clear instances of the progressive differentiation of ornament in time and in ontogeny. The Ordovician Cyathophycus reticulatus 28 is a turbinate form, with a rectangular mesh of longitudinal and transverse spicular rays (figure 37). At more or less regular intervals 50 STUDIES IN EVOLUTION some of the spicules are larger, thus dividing the surface into larger rectangular areas. In Dictyospongia prismatica 28 from the Devonian, the domination of eight of the longi- tudinal bundles of spicules has produced a prismatic form. D. Oonradi is regularly an eight-sided pyramid or prism when young, but with the growth and elongation of the sponge it developed slight undulations, then nodes, and later prominent tubercles (figure 38). Ceratodictya annulata and Hydnoceras nodosum 28 show a further specialization in the formation of rings and nodes. Practically the limit to these specializations is attained in Hydnoceras tuberosum 28 (figure 39), H. phymatodes, and related forms. In H. tubero- sum the apex representing the young stage or the initial growth is much like Cyathophycus or Dictyospongia. This is followed by a prismatic stage like D. prismatica and D. Con- radi-, then the nodes and tubercles are introduced and further growth produces the typical characters of the species. The tubercles are surmounted by a sharp spine formed at the intersection of two spicular laminse, one concentric and one longitudinal. Another type of surface specialization is shown in the genus Physospongia from the Keokuk group of the Lower Carboniferous. In this genus there are bands of regular, alternating, elevated, and depressed quadrules, the former frequently having the superficial layer of spicules extended into a spiniform process; as in P. Dawsoni. 28 Among corals there is occasionally some evidence of the external differentiation of structures into spines. The epi- theca of the Tetracorolla frequently shows, by means of low lines or low ridges, the number and direction of the septa, and in some of the later species these external septal lines are ornamented with rows of short spines or spinules; as in CyatJiaxonia cynodon 18 and Zaphrentis spi?iulosa. 18 Many Crinoidea and Asteroidea show the development of tubercles into spines, and the surface sculpture is often made up of ridges which bear strong spines at the points of intersection; as in Grilbertsocrinus tulerosus^ Technocrinus ORIGIN AND SIGNIFICANCE OF SPINES 51 spinulosus,^ Actinocrinus lobatusf 9 A. pernodosus, Greasier oceidentalis^ 0. gigas, Retaster cribrosus, etc. The concentric laminae of growth in the Brachiopoda are frequently differentiated into spinules; as in Siphonotreta unguiculata, 2 * 1 Schizambon typicalis^ 1 Spirifer fimbriatus^ 1 $. pseudolineatus^ 1 $. setigerus^ 1 Cliothyris Royssii^ 1 etc. Other species show the differentiation of the radii into spines; as Acanthothyris spinosa 1 * and A. Doderleini. lb In others the strong concentric laminae passing over radii are often infolded into spines; as in Atrypa spinosa. 31 Among the Mollusca innumerable examples could be cited showing clearly the differentiation of various ornamental features into spines. Some of these 40 have already been discussed, but may be referred to again in this connection. Thus an illustration of the passage of concentric laminae into spines is shown in Avicula sterna^ and Anomia aculeata 86 (fig- ures 26 and 28) and Margaritiphora fimbriata, etc. Many species of Gastropoda show the same types of differentiation. The differentiation FlGURB 4 o.-Zma sq ua- of radiating lines or ridges into *O*M*. Natural size. To show spines is equally common, and is well e aon of radii into shown in Spondylus (figures 12, 14, 30), and in Lima squamosus (figure 40). In most of these cases the rib represents the progression of a fold in the edge of the mantle, while the spine is a process of a con- centric lamina, and is usually more or less flat or tubular. Occasionally the rib becomes obsolescent, and is represented by a row of spines ; as in some specimens of the gastropod Crucibulum spinosum. When the radiating and concentric ornaments are distinctly continuous, a reticulate or cancellate appearance is produced, and the points of intersection often bear spines; as in Aviculopecten scabridus, A. ornatusf Actinopteria Boydi^ Pterinopecten spondylus etc. 52 STUDIES IN EVOLUTION The raised lines or ridges on the legs and carapaces of Crustacea are frequently spiniferous; as Crelasimus princeps, Grecarcinus ruricola, etc. The radii on the shells of barnacles are sometimes differentiated into spines ; as in Balanus tintin- nabulum var. spinosus. 18 In the higher animals the differentiation of ornamental features into spines is not common, especially as most of the forms are devoid of hard external parts. Among the fishes and reptiles certain lines and ridges on the head and body are often spiniferous, while in others the scales have spi- niferous ribs. III. Secondarily as a means of protection and offence. (A,, B 4 .) After spines have originated through the stimuli from the environment acting on the most exposed parts, or by growth force, or by progressive differentiation of previous structures, they may often acquire added qualities, one of which is to protect an organism from the attacks of many of its enemies. Morris 49 shows that defence in animals is either mechanical or motor, while in the higher plants it is purely mechanical. The spine clearly belongs to the mechanical mode of defence, and in many animals may be efficient without motion. If motion is added, it then may serve not only for protection but for offence as well. Natural selection evidently could not originate a spine, but after one has appeared from any of the causes mentioned in the preceding paragraph, this agency could tend to preserve and allow the spine to develop along certain lines. The restrictions as a defensive structure would be those of efficiency, and therefore all the monstrous growths, vagaries, and ornamental spine features would arise indepen- dently of the action of protective selection, and would be accounted for by the operation of the forces of the environ- ment, growth, and sexual selection. In this way the simple antlers of the Tertiary Deer may be imagined to have reached the highest degree of efficiency as weapons, by ordinary nat- ural selection (figure 41). In most cases the subsequent ORIGIN AND SIGNIFICANCE OF SPINES 53 increasing complexity of the antlers during more modern times cannot have improved their usefulness for protection or fighting (figures 42, 43), and probably arose through gradual specialization according to the law of multiplication of effects, acted on by the agency of sexual selection. In some species, as the Reindeer (Rangifer tarandus), the differ- entiation of the antlers has secondarily produced a useful structure. One of the brow tines in this species has become greatly enlarged and palmated, and serves to assist in remov- ing the snow to uncover food. Evidently this has had some- thing to do with the common retention of the antlers in both sexes. 41 42 43 FIGURE 41. Antler of Cervulus (?) dicranoceros. Pliocene. FIGURE 42. Antler of Cervus pardinensis. Pliocene. FIGURE 43. Antler of the Fallow Deer (Cervus dama). Reduced. Nicholson and Lydekker.) (After Certain types of horns are common to particular regions, especially when the cattle are in a semi-wild state, as in the Western Plains of America. The Texas cattle have long, gently curved horns standing out from the head. Similar forms are prevalent in the cattle of southern Italy and in other warm temperate regions. Further north, the horns become more curved in a direction parallel with the head, and are therefore closer to the skull. The most northerly representative of the hollow-horned ruminants, the Musk-Ox (Ovibos moschatus), has the horns hanging down close to the skull and only curved outward in their distal portions. 54 STUDIES IN EVOLUTION Marsh suggests to the writer that these variations in the directions of the horns have been influenced by the climate. A warm climate permits the horns to stand out directly from the skull. Further north, or in a colder region, the frequent freezing of the horns and their consequent drooping has induced a natural drooping condition, and an Arctic climate has resulted in the production of horns closely appressed to the skull, in which position they cannot be affected by freez- ing temperatures. Another possible service for antlers is also suggested by Marsh. As is well known, the male Moose is one of the most wary of the Cervidse, and detects noises at great dis- tances. The large palmate antlers act as sounding-boards, and, when listening, the animal holds his ears in the focus of the anterior surfaces of the antlers. The hollow-horned mammals afford some of the most evi- dent examples of the use of horns for protection and offence. In species with permanent horns, like the bison, oxen, goats, cattle, antelopes, etc., the horns are generally present in both sexes, though in the males they are often much the larger. In defence, many of the horned ruminants hold the head down, thus protecting the nose and bringing the top of the skull into prominence. In this position the horns are most effective. A similar posture is taken by the horned batra- chians and lizards. The Porcupine and Echidna rely largely on the protection afforded by their spines, and on this account they are sluggish in their movements, and make little effort to escape approach- ing enemies. Many of the great horned Dinosauria of the Mesozoic are well provided with an armature of protective plates and spines on various parts of the body. In addition to an armature on the body Triceratops^ had three large horns on the head, one median (nasal) and two lateral (supra-orbital). These were powerful offensive and defensive weapons. There were also other small nodes and spiniform ossicles around the posterior crest of the skull and on the jugals, forming a part ORIGIN AND SIGNIFICANCE OF SPINES 55 of the general armor. In Stegosaurus 46 the efficient offen- sive and defensive weapons were the huge spines on the tail, and it is interesting to note, as a parallel to this condition, that the greatest nerve centres were in the sacrum, and therefore posterior also. No group of vertebrates shows such a variety of protective and offensive characters as the fishes. Many of the older types were heavily plated, while in others the fin spines were greatly developed. Among modern forms the protective character of the spines is well shown in types like the Spiny Box-fish Chilomycterus geometricus and Diodon maculatus. A combination of mechanical and optical protection is afforded in the remarkable Australian Pipe-fish Phyllopteryx eques 2 ' (figure 49). This fish has numerous spines and ribbon-like branching filaments, the former giving it a mechanical defence, and the latter assisting in its concealment among sea-weeds, to which it bears a striking resemblance. Spines for protection are extremely common among insects, even in larval forms. They have been so frequently noted as to require no elaboration here. Packard 54 has ably discussed the origin of nodes, tubercles, and spines among certain caterpillars. Among the forms which feed exclusively at or near the ground, he finds the body usually smooth, while those feeding on trees or on both trees and ground are often variously spined and tuberculated. These ornamental features arise from the modification of the piliferous warts common to all lepidopterous larvae, and he concludes that the trees were more favorable for temperature, food, etc., than the ground, and that an increase of nutrition and growth force led to the hypertrophy of these warts into tubercles and spines. Having thus arisen, they immediately became useful for protection from birds and parasitic insects. Among the Crustacea there are also numerous examples of protective spines. These may be confined to parts of the body and legs especially exposed, or the entire animal may partake of the spiny character, as in the crab Echidnoce- ras setimanus, where even the eye-stalks and antennae are 56 STUDIES IN EVOLUTION spiniferous. Others, like Lithodes maia, have the spines generally distributed over the carapace and legs. While serving for defensive purposes, this generally spinose char- acter has probably reached its extreme development through the influence of repetition (B 2 ). The nauplius larva of Lepas fascicularis is very large, and has highly defensive spines which are explained by Balfour 8 as a secondary ad- aptation for protection. The larger spines on Trilobita, especially those from the genal angles and the axis, doubtless served protective purposes. The extremes of spinosity in FIGURE 44. Zoea of the common crab (Cancer irroratus) ; lateral view. X8. (After Verrill and Smith 68 .) this class are found in the various species and genera of the family Acidaspidae, and also in many forms of Arges, Terataspis, Hoplolichas, etc. Even among the star-fishes, which are so generally spinose, some forms have the spines so prominently developed on the most exposed portions of the animal that they evidently serve for protection ; as Acanthaster Solaris, JSchinaster spinosus, etc. The examples already given are sufficient to emphasize the fact that after spines are developed they may then often serve for protection and offence, and therefore be useful, their efficiency being controlled by natural selection resulting in the survival of the fittest. ORIGIN AND SIGNIFICANCE OF SPINES 57 Another process or kind of selection has been described by Verrill as "Cannibalistic Selection." He has shown that the young of carnivorous animals often prey upon each other, as in the larval forms of some Decapoda, or sometimes even before the escape of the young from the egg capsules, as in some of the Gastropoda. Here, of course, any natural variation in the newly hatched animals which would give an individual some advantage over its companions would tend to its preservation and to their destruction. In this way it may occur that the relative growth of spines in the zoe'a of decapods has determined the survival of the well-armed individuals; as in the zoe'a of Cancer Q * (figure 44), Carcinus, Homarus, etc. IV. Secondarily from sexual selection. (A 4 , B 4 .) The males and females of so many animals present dif- ferences in size, color, and ornament, that corresponding variations in the development of spines, horns, and antlers, might naturally be expected. That such differences actually occur in nature is evident. Every gradation can be found between horns or antlers common to both sexes and those confined to one sex. Probably the initial difference is as ancient as sex itself. Sexual variations of horns are most familiar among the mammals. Some, as the Giraffe, Ox, Bison, and Reindeer, have them present in both sexes, though the antlers of the female Reindeer are smaller and more slender than in the male, and in the American variety are sometimes absent. Others, as in the Prong-horn Antelope, many sheep, goats, etc., have the horns usually quite small in the female, and well developed in the male. Lastly, the modern Deer, Elk, Moose, etc., have the antlers confined to the males alone, the female being entirely without them. Some of the early deer (Procervulus) seem to have had antlers in both sexes, and in nearly all the families of the 58 STUDIES IN EVOLUTION Ruminata there are species without horns, other species with horns in both sexes, and still others with horns only in the male. In the wild state the presence or absence of horns and their character in any particular species seem to be well established, but in domesticated forms the greatest variety is found. Among domesticated cattle, presumably of one species originally, varieties are found without horns, and others , with horns showing all degrees of twisting and length. tj&r By protecting cattle from enemies, by forcing them into }^ A, changed environment, and by varying amounts of nutrition, $+' man has evidently brought the original stock into a condition j/^ of free variation. This state has been made use of in the production of endless varieties by selection and cross-breeding. Darwin 14 accounts for the sexual selection affecting the growth of the antlers in the Deer as due to excess in the number of male individuals, and their struggles for supremacy in the possession of a mate. The antlers at the breeding season are strong and solid, and are therefore at their maximum of efficiency in each individual. They are shed at or before the time the young are born. Previous to the growth and maturity of the new antlers, the young are so far advanced as to be able to avoid being killed by the adult males. Furthermore, Darwin suggests that the excessive * i development of antlers into palmate and arborescent forms was probably an ornamental character attractive to the females. These complicated antlers not being the most efficient weapons, the fighting proclivities of the males would tend to favor the individuals with simple antlers, and to repress the more differentiated forms. Thus the two in- fluences would be opposed to each other, though not necessarily equal. The law of the multiplication of effects may also have some force, since it may carry a structure beyond the bounds of efficiency. Even in one of the oldest horned mammals, the Protoceras 45 of the Miocene Tertiary, a great difference is seen in the horns of the two sexes. The female has little nodes or tubercles, which in the male rise to the ORIGIN AND SIGNIFICANCE OF SPINES 59 height and prominence of the horns on the Giraffe, or are even relatively more pronounced. The males of some other vertebrates have spiniform processes or spurs on their legs and wings serving particular functions. The spurs in birds are to be considered mainly as weapons which are used by the males in combats among themselves. They are developed on the metatarsal or metacarpal bones as bony processes ensheathed in horn. In the females the spurs are generally rudimentary. A kind of spur is also found on the hind limbs of the male Echidna and Ornitho- rfiynchus, attached to the astragalus. It is perforated by a duct leading from a gland. The functions of the spur and of the secretion are unknown. Many lizards, especially among the Chamseleontidse, present striking differences between the sexes, and the males of some of them develop veritable horns like those in cattle, sheep, and other hollow-horned ruminants. Darwin 14 illustrates and describes a number of most interesting examples. One of them Chamceleon Oweni is tyere shown (figures 45, 46). The male has three horns, one on the snout and two on the - forehead. They are supported by bony excrescences from the skull. From the peaceable nature of these animals, Darwin concludes that "we are driven to infer that these almost monstrous deviations of structure serve as masculine ornaments." The males of the tropical American genus of fishes Oal- lichthys "have the spines on the pectoral fins stronger and longer than those of the female, the spine increasing in size as the male reaches maturity" (Seeley 65 ). Among insects the males of many beetles belonging to the lamellicorns have long horns arising from various parts of the head and thorax. One of the best known forms is the Hercules beetle Dynastes hercules. Bateson 5 states that, in this and other genera, it is commonly found that the males are not all alike; but some are of about the size of the females and have little or no development of horns, while others are more than twice the size of the females and have 60 STUDIES IN EVOLUTION 45 enormous horns. These two forms of male are called " low " and "high" males, respectively. Among the males sim- ilar dimorphism in respect to size and length of horns occurs in Xylotrupes gideon, and in the stag beetle (Luca- nus cervus, L. titanus, L. dama). In many of these cases the horns are evidently protective, and not de- veloped through the selective influ- ences of the female. In such cases the habits of the male are supposedly different from those of the female. Thus Wallace 70 suggests that the horned males of the coleopterid fam- ilies Copridse and Dynastidse fly about more, as is commonly the case with male insects, and that the horns are an efficient protection against insectivorous birds. These interpretations clearly do not come under the definition of sexual selec- tion as restricted to the choice of either sex. Beauty, voice, or strength may influence the selection of a mate by the opposite sex, but when the habits of the sexes are different, and certain characters arise in response to this change, the explanation is then really found in the law of adaptation or physical selection. FIGURE 45. Profile of head of Chamceleon Oweni ; male. \. FIGURE 46. Female of the same species. \. (After Darwin.) V. Secondarily from mimetic influences. (A 5 , B 4 .) Natural selection may aid in furthering and preserving a spinose organism after the spines have originated through any primary cause. One aspect of this influence may be treated under the head of mimicry. If, by their resemblance in form, color, or voice, any characters are similar to char- acters present in the surroundings of the animal, and afford a means of protection or are useful, they may be considered as mimetic in the broadest sense of the term. Mimicry is ORIGIN AND SIGNIFICANCE OF SPINES 61 usually restricted to a kind of special resemblance, and not to the cases of general resemblance afforded by an animal without significant colors in general harmony with its surroundings. The influence of mimicry in the production of spines can only occur where the object mimicked is spiniform or spinose. Apparently this is rather infrequent and of little real impor- tance as a factor of acanthogeny. Insects and spiders have furnished the greatest number and variety of mimetic forms, both in their larval and adult conditions, and naturally would be expected to furnish ex- amples of spines having mimetic significance. The object mimicked may be another species of insect or animal, in which case there is usually some offensive or defensive quality rendering the resemblance useful to the mimicker; or the whole or a portion of some plant or other object may be imitated, tending to the more or less complete conceal- ment of the mimicking insect. Satisfactory examples are not at hand, though doubtless many occur in nature, and some Jiave been described, but not for the present purpose. A few will be cited here which seem to conform to the requirements. 47 FIGURE 47. Profile of a spider (Ccerostris mitralis) on a twig mimicking a spiny excrescence. (From Peckham, after Vinson.) FIGURE 48. The larva of the Early Thorn Moth (Selenia iHunaria) resting on a twig; showing mimicry of stem and spiniform processes. ^. (After Poulton.) A Madagascar spider (^Ccerostris mitralis) is described by Elizabeth G. Peckham 66 as sitting motionless on a branch 62 STUDIES IN EVOLUTION and resembling a woody excrescence with projections or spiniform processes (figure 47). Other spiny spiders of the Epeiridse probably have similar protective mimetic features ; as Epeira spinea and Acrosoma arcuata. The larva of the Early Thorn Moth, as described and illus- trated by Poulton, 58 bears a strong resemblance to the twig upon which it rests, even to the spiniform processes, axils, and buds (figure 48). Packard 54 cites a striking case of mimicry 49 FIGURE 49. Australian Pipe-fish (Phyllopteryx eques) and frond of sea-weed in lower right-hand corner ; showing mimicry. . (After Gunther.) in the caterpillar of another genus of moth (ScUzurd), where the spines and tubercles resemble the serrations of a leaf "so that, when feeding on the edge of a leaf, the Schizurse exactly imitate a portion of the fresh-green serrated edge of a leaf including a sere, brown, withered spot, the angular, serrate outline of the back corresponding to the serrate out- line of the edge of the leaf." The Australian Pipe-fish Phyllopteryx, previously men- tioned under the head of spines for protection, shows the ORIGIN AND SIGNIFICANCE OF SPINES 63 mimicry of a plant by an animal to a striking degree. This fish closely imitates a sea-weed (figure 49), and Giinther 25 gives the following description of the spines and filaments on the species Phyllopteryx eques : " There is a pair of small spines behind the middle of the upper edge of the snout, a pair of minute barbels at the chin, and a pair of long appen- dages in the middle of the lower part of the head. The forehead bears a broad, erect, somewhat four-sided crest, behind which there is a single shorter spine. A horizontal spine extends above each orbit. There is a cluster of spines on the occiput, and from these narrow appendages are pro- longed. On the nape of the neck is a long spine, dilated at the base into a crest, and carrying a long forked appendage. The back is arched, and on the under side are two deep indentations. The spines on the ridges of the shields are the strongest; they are compressed, are not flexible, and each terminates in a pair of short points. There is one pair of these spines in the middle of the back, and one on each of the three prominences of the abcjominal outline ; they termi- nate in flaps, which are long and forked. There are also very long compressed flexible spines without appendages, which extend in pairs along the uppermost part of the back, while a single series extends along the middle line of the belly. Small short conical spines run in a single series along the middle line of the sides, and along the lateral edges of the belly ; and there is a pair of similar spines in front of the base of the pectoral fin. The tail, which is about as long as the body, carries the dorsal fin ; it is quadrangular, and has sharp edges. It carries along its upper side five pairs of band- bearing spines, which terminate in branching filaments." The Horned Toad Phrynosoma bears considerable resem- blance to the joints of the Prickly Pear, with which it is often associated, and it may be suggested that the likeness both in form and spinescence represents mimetic characters. * The artist who copied Giinther's figure for Leunis' " Synopsis der Thier- kuude," 3d ed., by H. Ludwig (vol. i. p. 770, 1883), connected the fish with the adjacent fronds of sea-weed so as to form a single organism. 64 STUDIES IN EVOLUTION VI. Prolonged development under conditions favorable for multiplication. (Bi.) The prolonged development or existence of a stock under favorable conditions for multiplication may be considered as one of the primary influences favoring the production of spines. This implies abundance of nutrition and compara- tively few enemies outside of other individuals of the same or closely related species. Under a proper amount of increased nutrition the vitality and reproductiveness of a stock are raised, and, other things being favorable, it is found that the stock will give expression to what has already been described as free variation. Hypertrophy is also very apt to be one result of abundant nutrition, so that structures of little or no use may be developed, and some of them comprise certain features which are often called ornamental. In the excessive multiplication of individuals it is evident that there must be a great number of natural variations, and that some of these will affect the pairing of the sexes in such a manner as to accentuate and delimit certain variations. Eventually there also comes a struggle for existence in which favorable modifications have a decided advantage. In this way it is believed that the great amount of differen- tiation found in some isolated stocks has been brought about. Primarily, then, a favorable condition for nutrition is assumed, which is followed by excessive numerical multi- plication; while the natural variations are augmented and governed by the action of reproductive divergence for which such conditions are favorable. Secondarily, these variations are subjected to the influences of cannibalistic selection, defence, offence, sexual selection, and mimicry. In illustration of the amount of differentiation attained by a single stock under favorable conditions, the Amphipod Crustaceans G-ammarus and Allorchestes, found in lakes Baikal and Titicaca, respectively, may again be noticed. In respect to the number of species, G-ammarus is very sparsely distributed over the world, though in Lake Baikal ORIGIN AND SIGNIFICANCE OF SPINES 65 alone a hundred and seventeen species have been described "^J by Dybowsky. 17 In contrast to this, it may be mentioned that but four freshwater species have been discovered in the whole of Norway. In Lake Baikal all the depths explored (to 1,373 metres) have furnished species. Those living near the surface are vividly colored, yet apparently make no attempts at concealment. Many of the species are also highly spinose, though not sufficiently armed to be protected from the fish. As these Crustaceans are voracious creatures, the spinose character has probably been favored by the agency of cannibalistic selection. The lake has a number of species of fish for which the Gammaridse furnish excellent food, but the presence of a species of seal, predaceous fish, as well as the native fishermen, keep the fish below the danger point, thus allowing the GammaridaB to become very abundant. Similarly, in Lake Titicaca there 50 is a wonderful specific development of a kindred Crustacean Allorches- tes. One of the most spinose spe- cies (A. armatus) is also the com- monest, and according to Faxon 19 occurs in countless numbers (fig- ure 50). Packard 54 shows that, among certain moths, the caterpillars as soon as they acquired arboreal hab- its met with favorable conditions in respect to food, temperature, etc., and that as spines and tuber- FIGURE 50.- Alhrchestes ar- cles arose by normal variation, such matus, a spiny amphipod from features, being found useful for protection, were therefore preserved (After Faxon.) and augmented. The differentiation of Achatinella has already been dis- cussed (p. 36) as affording a striking instance of free varia- tion among the Mollusca. The evolution of the Tertiary species of Planorlis at Steinheim, as described by Hyatt, 35 5 66 STUDIES IN EVOLUTION furnishes another example, though in neither case has the differentiation of structures proceeded far enough to result in spines. The costate form (Planorbis costatus) was tending toward that end, but did not attain it. The series of Slavonian Paludina in the Lower Pliocene, as elucidated by Neumayr and Paul, 50 shows a somewhat further advancement. The species in the lowest beds (typus Paludina Neumayri) are smooth and unornamented. Higher in the strata they are angular and carinated, and at the top of the series the shells are carinated, nodose, and sub-spinose (typus Paludina Hoernesi). The living American genus Tulotoma is closely related to the most differentiated species (P. Hcernesi), and its approach to spinose features is more pronounced. Under the phylogeny of spinose forms (pp. 23-25) an outline of the life history of the brachiopod Atrypa reticularis and derived species was presented. This being one of the commonest types of Brachiopoda in the Silurian and Devo- nian, often forming beds of considerable extent, it seems quite likely that its prolonged development under favorable conditions for multiplication must have had an effect on the amount and kind of variation. It has been noticed by Brady 9 and others, that in the Foraminifera, Crlobigerina bulloides, Orbulina universa, etc., the pelagic forms comprise two varieties which are generally distinct, a spinous form and another with small minutely granular shells. The bottom specimens of the same species are also commonly without spines and often smaller. The interpretation seems to be that the large specimens indicate an abundance of nutrition which has also produced hyper- trophy of the normal granules into spines. Some bottom specimens are large, but they are usually abnormal and of a monstrous or pathologic nature. From the foregoing examples the conclusion to be drawn is that, with full nutrition, there comes a numerical maxi- mum, and naturally with this a corresponding number of normal variations. Some of these modifications, as spines, ORIGIN AND SIGNIFICANCE OF SPINES 67 have arisen by hypertrophy. After having thus originated by growth force, they may or may not be of use for offence, defence, or concealment, or in any way give their possessor a distinct advantage. VII. By repetition. (B 2 .) Under the consideration of spine production by repetition it is proposed to include local repetition or duplication of spines on or about a primary spine, the limit of this repetition resulting in a generally spinose condition. It has been shown that intermittent stimulus produces growth, and furthermore that growth can take place only with proper nutrition. Under local stimulus the currents of the circulation or forces of nutrition are set up in an organism toward the centre of stimulation. The nutrient matter is brought to this point, and more or less of it is expended in building up a structure which is the reciprocal or direct resultant of the stimulus. Now, since all motion is primarily rhythmic, 66 and the repetition of parts an almost universal character among organisms, 5 it would appear that the fore- going conditions would be favorable to the repetition or reproduction of the structures. In this way it is easy to account for the growth of spines that cannot be explained as the direct result of external stimuli (A), or by any process of decrescence (C, D). The nature of the influence seems to be similar to induction in electrical physics, or to the force or stimulus of example in human conduct. Stated as a concrete case, a simple spine produced by any primary cause may be taken, and it will be granted that the vital or physiological adjustments produced in its growth and maintenance have brought about or induced an harmonic condition in the adjacent tissues. Subsequent growth will most naturally repeat the previous structures, so that in addition to the primary spine there will be other smaller spines on or about it, together constituting either a com- pound spine or a group of spines. 68 STUDIES IN EVOLUTION Carrying this repetitionary process to a maximum, there would result a generally spinous condition. As a possible illustration of this, no class of organisms probably exhibits so many kinds and series of repetitions of all sorts of external structures as the Echinodermata, and it is significant that this is a typically spiniferous sub-kingdom. Except in a few classes of organisms, compound spines are relatively rare as compared with simple spines. They are very common among the Radiolaria, which furnish the greatest complexity occurring anywhere in the organic world. (See Plate I.) They are also quite frequent among the Echinoidea, but more rare among the Asteroidea and Crinoidea. Compound antlers are especially characteristic of the mod- ern Deer family, though compound horns are but rarely found elsewhere among the mammals. The Prong-horn Antelope of America is the only living species of hollow- horned ruminant having this character. It of course is not intended that extra pairs of horns, which being separate, and often originating on different portions of the skull, should be considered as compound horns in the sense here employed. Likewise compound spines arising through suppression of organs or structures are not to be included here; as the compound thorns on the Honey-locust representing aborted branches. The fin spines of fishes are often compound, and sometimes are made up of several elements ; as in the spines of Edestus (E. vorax). Quite a number of Mollusca develop compound spines ; as in many species of Spondylus and Murex. They are also not uncommon among the Crustacea and Insecta. Compound spines are infrequent in the Brachiopoda, being developed in but few species {Spirifer hirtus 31 ). The Foraminifera also present but few examples (Polymorphina Orbignii 9 ). A number of generally or highly spinose types will now be noted to illustrate the limits of the repetition of spiny structures, the first spines having probably arisen through ORIGIN AND SIGNIFICANCE OF SPINES 69 the operation of some primary cause, and the derived or secondary spines being produced, it is believed, by the law of repetition. The Radiolaria have already been frequently mentioned, but as they are the most spiniferous of all classes of animals, and represent the highest degree of spine differentiation attained (figure 51 and Plate I), another brief notice will 51 52 FIGURE 51. Acontaspis hastata, a radiolarian ; showing multiplication of spines by repetition. X 200. (After Haeckel.) FIGURE 52. Strophalosia keokuk, an attached brachiopod ; showing the spines extending from the ventral valve to and along the surface of attachment. X2. FIGURE 53. A gastropod shell (Platyceras) to which are attached a number of Strophalosia keokuk. Natural size. be of interest. These spines furnish characters of high taxonomic value, although generally speaking they seldom have more than specific importance among other classes. The Echinoidea and Asteroidea must also be noticed in this connection, though from the nature and origin of their spines they do not conform to the mode of spine growth in other classes. Productus, Productella, Strophalosia^ Aulosteges, and Sipho- 70 STUDIES IN EVOLUTION notreta represent highly spinose genera among the Brachi- opoda. Strophalosia is a form in which the ventral valve is cemented to some object. Whenever the valve rises well above the object of support, the spines are free like those frequently present on the dorsal valve ; otherwise the spines extend root-like along the supporting surface (figures 52, 53). Aulosteges presents a still further tendency to complete spinosity, for not only are both valves covered with spines, but the deltidium also. Spondylus (figure 30) and Murex are well-known types of very spiny forms of Mollusca. Acidaspis, Terataspis, etc., hold the same place among the Trilobita; Ecliidnoceras, Lithodes, etc., among the Decapoda; and the Spiny Box-fish (Diodori), Pipe-fish, etc., among the Pisces. The higher animals also furnish examples of extreme spinosity; as in the Horned Toad (Phrynosoma), the genera of Ceratop- sidee, gigantic Cretaceous Dinosauria, and the Echidna and Porcupine. All these forms present numerous spines, some of which cannot be explained as having arisen directly from external stimuli, for they are in comparatively well-protected regions out of the way of external stimuli. Neither can all of them serve for offence and defence, as they are often not located in the most advantageous positions; nor are they differen- tiated out of any previous ornaments or special structures. In fact, no factor of spine genesis except the one of repetition seems to be sufficient to account for their development. VIII. Restraint of environment causing suppression of structures. (Ci.) The previous categories of spine production (I- VII) have been brought about by some process of growth or concres- cence through external and internal agencies. There still remains for discussion the formation of spines by processes of decrescence caused by extrinsic restraint (C) or by intrinsic deficiency of growth power (D). The lack of vitality or ORIGIN AND SIGNIFICANCE OF SPINES 71 growth force generally stands so directly as the result of an unfavorable environment, that it is often difficult or impos- sible to distinguish between their action. Furthermore, as in the case of many parasites, it may be seen that the envi ronment may be quite favorable as regards temperature, nutrition, etc. ; but unfavorable in respect to motion and use of sensory and motive organs. From the almost universal degradation and retrogression of parasitic forms, it is neces- sary to consider these as intrinsically deficient, and therefo lacking in the qualities ofgrowtETIbrce which normally favor^. a progressive evolution. Here, also, there are apparently two intimately associated causes. In an attached animal the absence of stimulus from disuse of an organ tends toward atrophy, and the retrogressive development serves to affect many organs in the same manner. The direct and indirect results of the restraint of the environment may therefore be expected to shade imperceptibly into each other, with only the extremes sufficiently distinct for separation. The influence of an unfavorable environment as affecting the character and growth of plants and animals is well shown in desert or arid regions, and the flora has been made the subject of especial study by Henslow. 83 In such regions the first thing to impress the observer is the small size of the species. Next to diminutive size, the scantiness of life is a striking feature, for large areas are common in which life is almost wanting. An examination of these plants reveals a series of characters not usually present elsewhere, among which may be mentioned the development of a minimum amount of surface, constituting what is known as consoli- dated vegetation; next their uniform gray color, due either to excessive hairiness or a coating of wax ; and lastly, their frequent spinescent characters. The spines on desert plants are a feature of such general occurrence that it has led to the notion that vegetable spines are always associated with unfavorable conditions and are therefore suppressed structures. This is probably incorrect* 72 STUDIES IN EVOLUTION for in plants, as in animals, spines may be developed by the progressive differentiation of previous structures; as in the angular edges of the leaf stems of many Palms becoming spiniferous, or, as will be shown, suppressed structures may arise from deficiency of growth force. In all cases spines may or may not serve for protection. Thus, while they are not always an indication of unfavorable environment, those occurring on desert plants may generally be so considered, for they are developed out of structures which are normally of vital physiological importance. An animal or plant having spines and living in a favorable environment, involving freedom of motion for animals and abundance of nutrition without extremes of temperature or dryness for both animals and plants, will, it is believed, from the discussions and analyses of spine genesis in its various phases, develop these features in most instances with- out the sacrifice of organs and structures having important physiological and motor functions. Thus, ordinarily, among animals it is found that spines arise as excrescences or out- growths of exoskeletal or epidermal tissues, without seriously affecting the function of the organ or organs upon which they are located. Such cases may clearly belong to the most progressive series, and in fact usually occur there. On the other hand, if it is found that a leg, a wing, a digit, or other organ is developed into a spine, this is always accomplished by a process of retrogression, resulting in the greater or lesser suppression of the part in question. It is also seen that this kind of spine occurs most frequently in retrogressive series or in others showing arrested develop- ment, and the necessary interpretation seems to be either that the environment is or has been unfavorable, at least so far as the particular organ or set of organs is concerned, or that the vital power has declined. Both influences are intimately associated, and the latter is often the direct result of the former. The stunting effects of aridity and barren soil on com- mon plants is familiar to all. Among the plants of the ORIGIN AND SIGNIFICANCE OF SPINES 73 desert is found every evidence of similar stunting combined with adaptations to resist the unfavorable conditions of defi- cient water supply, excess of radiation, etc. The diminution in size applies not only to stature, but to the leaves and branches, especially the parenchymatous tissues or parts of the plant engaged in aerial assimilation. Consonant with these changes, the drought and other conditions produce a hardening of the mechanical tissues, which is of great aid in resisting the extreme heat and dryness of the desert. Some- times a deposit of wax affords a similar protection. The reduction of the leaves takes place in various ways. They may simply become smaller in every dimension and finally be reduced to mere scales, or an aphyllous condition may be established. They may grow narrower and narrower until only the hardened veins or midrib remain; or leaves may be developed only for a short time, and in the case of compound leaves after the shedding of the leaflets a spini- f orm leaf axis remains ; as in Astragalus Tragacantha (figures 55, 56). The suppression of ., branches tends toward the same end ; namely, either to their complete disappearance or to their partial suppression into hard spiniform processes or thorns. Thus leaves, branches, and other parts of the plants may become reduced to their axial elements, bringing about what is commonly termed spinescence. The spiny character of these plants is therefore one of the results of an arid environment, and it may or may not be of sufficient frequency to give an especial character to a partic- ular desert flora. There is, moreover, a secondary influence which has an effect in determining the abundance of spinose plants in desert as well as in many other situations. This relates to the destruction of the edible unarmed species by herbivorous animals, and the comparative immunity of the spiny types. Thus, in old pastures, the prevailing flora is apt to be one that is offensive to grazing animals. This character is generally given by poisonous plants or those having a disagreeable flavor, or by those whose form or spiny structures afford protection. 74 STUDIES IN EVOLUTION This secondary influence by grazing animals may have had some effect in determining the particular abundance of spiny plants in certain desert regions, and their comparative infre- quency in other similar regions. In either case the unfavor- able environment brings about a suppression of structures, and one type of this action results in the production of spines. These represent the limits of retrogression before the part becomes entirely obsolete. Wallace has criticised Henslow's views on the origin of xerophilous plants and their distribution. It is believed that the views here offered remove some of the objections, and bring the opinions of these authors into greater accord. Under arid conditions bracts, stipules, leaves, and even branches may become spinescent. Some forms in which the spinose character has not as yet become permanently fixed by heredity, when transported or found living in moister and richer soils, develop normal leaves or branches, and lose their spinescence ; others, like the Cactus, retain their spines under similar changes; while still others, as Acanthosicyos hor- rida,^ cannot be artificially cultivated, and have become truly xerophilous types. As examples of plants which lose their spines by cultiva- tion, the Pear, species of Rose, Plum, etc. (Henslow), may be cited. According to Henslow, 83 others, as Onomis spinosa, have an especially spiny variety (horrida) living on sandy sea-shores, while in more favorable natural situations the same plant becomes much less spiny, and under cultivation loses its spines. M. Lothelier 42 also found that by growing the Barberry (Berberis vulgaris) in moist air, the spines dis- appeared, the parenchyma of the leaves being well formed between the ribs and veins. Dry atmosphere and intense light both favored the production of spines. Henslow 33 cites the genus Zilla as a desert plant in which the branches are transformed into spines, EMnops for a similar modification of the foliage, Fagonia for spiniform stipules, and Centaur ea for spinescent bracts. As further illustrations taken not only from desert plants but also from ORIGIN AND SIGNIFICANCE OF SPINES 75 others commonly found in dry, rocky, or unfertile situations, the following examples may be taken, some of which are familiar cultivated species: The stunting of branches into spines is common among neglected Pear and Plum trees, and is a normal character in the Hawthorn, Honey-locust, Oytisus (figure 54), Vella, etc. Leaves transformed into spines are characteristic of the Cactacese of America, the columnar Euphorbiacese of Africa and southern Asia, and are also familiar in the half-shrubby Tragacanth bushes (figures 55, 56) so common in southern Europe, especially in the eastern portion, and in the ordinary Barberry (figure 13). Spiniform stipules are usually present in the species of Rolinia, of which the Common Locust (Robinia Pseudacacia) furnishes a well-known illustration (figure 57). Spiniform bracts are best known among the Thistles (CHrsium lanceo- latum, C. horridulum, etc.). 54 55 56 57 FIGURE 54. The spiny Cytisus (C. spinosus) ; showing suppression of branches into spines. (After Kerner.) FIGURE 55. A single leaf of Tragacanth (Astragalus Tragacantha), from which the three upper leaflets have fallen. (After Kerner.) FIGURE 56. Leaf axis of the same, from which all the leaflets have fallen. (After Kerner.) FIGURE 57. Twig of Common Locust (Robinia Pseudacacia) ; showing spines representing stipules. As the restraint of an environment acting on an animal so generally results in the disuse and atrophy of the organs affected, most cases will have to be considered under the head of disuse. Therefore, while the environment is the 76 STUDIES IN EVOLUTION primary factor, its influences are mainly exhibited through secondary or resultant conditions. In some cases, however, it is possible to interpret a vestigial or suppressed structure directly into terms of an unfavorable environment. Thus, if the probable origin of the vestigial hind legs of a Python is considered, it leads to the belief that they represent legs which were of functional importance to some of the early ancestors of this snake. The gradual elongation of the body and the consequent change from a walking or direct crawling habit to a mode of progression chiefly by horizontal undula- tions, necessarily brought the legs into a relation with the environment which was unfavorable either for their function or growth. Their suppression is complete in most snakes, but in the Python the hind legs are represented by two spurs or spines (figures 58 and 59). On the interior of the body they are supported by vestiges of femora and ilia, showing their true affinities with hind limbs. Some snake-like batra- chians, as Amphiuma and Proteus, still retain short and weak external limbs. These would undoubtedly soon be lost by a change from aquatic to terrestrial or arboreal habits. 58 59 FIGURE 58. Portion of skin of Python ; showing the spurs which represent the suppressed or vestigial hind legs. X \- (After Romanes.) FIGURE 59. Bones of suppressed legs of Python. All but the claw-like termination are internal. X \. (After Romanes.) In explanation of the nodes and spiniform processes on the epitheca of Michelinia favosa, it may be suggested that they represent aborted corallites or attempts at budding. This coral belongs to the order Porifera, which has been shown ORIGIN AND SIGNIFICANCE OF SPINES 77 by the writer 7 to have very pronounced tendencies toward proliferation, and on the interior of the colony these attempts result in the production of mural pores. Most of the species of Michelinia are hemispherical or spherical. M. favosa is inclined to be pyriform in shape, rising above the object of support, and thus presenting a rather large epithecal surface. Manifestly the lower side of the corallum is unfavorably situ- ated for the growth of corallites, and any efforts at prolifera- tion on the part of the peripheral corallites is apt to result in stunted outgrowths. There is here a very close connection between restraint of environment and deficiency of growth force. If the whole corallum is taken into consideration, the restraint of the environment may be taken as preventing the growth of corallites on the lower side. If one of these single stunted corallites is considered, it may be said that the defi- ciency of growth force through lack of nutrition caused its suppression. IX. Mechanical restraint. (C 2 .) Among the factors of spine genesis mechanical restraint is probably of the least importance. It can only rarely happen that an organism is forced to grow a spine contrary to the natural tendencies of normal development. Yet, as there are occasional types of spiniform structures which can be best explained as due to the mechanical restraint of the environ- ment, it is necessary to notice them in order to make the categories of origin as complete as possible. The illustrations will be taken chiefly from the Brachiopoda and Trilobita. The recent brachiopod Muhlfeldtia truncata is semi-elliptical in outline, and has a very short stout pedicle which holds the shell so closely to the object of support that the beak is truncated from abrasion and resorption. In specimens attached to a small branch of a coral, thus allowing the cardinal extremities of the shell to project beyond the object of support, the ends of the hinge are generally rounded. Specimens growing on a large flat surface have the cardinal extremities angular or sub-mucronate. Similar variations are 78 STUDIES IN EVOLUTION to be observed in other living species of Brachiopoda (Cfo- tella,) some Dallina, etc.)- Some of the extinct genera show more highly developed cardinal extremities which are often very characteristic of certain species, though considerable variation is found to exist. It is evident that these elon- gated hinge-lines have arisen from the mechanical necessities of a functional hinge, and their greater or less extent is also to a degree dependent upon the nature of the object of sup- port, which furnishes a stimulus to the growing ends of the hinge. A marked example is shown in Spirifer mucronatus, 60 with the cardinal angles extended into spiniform processes (figure 60). Similar features are presented by many FIGURE To. - Dorsal ther S P 6cieS f Spirifer, Orthis, Lep- view of Spirifer mucrona- tcena, Stropheodonta, etc. tus; Devonian; showing In the Trilobita the pygidium, or spiniform cardinal angles. . . . . X f. (After Hall and abdominal portion, consists of a num- Clarke.) ber of consolidated segments, and the segments of the thorax are successively added in front of this tail piece. The first thoracic segment is therefore formed between the cephalon and pygidium, and its form is mechani- cally in agreement with the requirements of the animal for bending the body, and with the adjacent margins of the cephalon and pygidium. In a way it may be said that the segment is moulded by the adjacent _parts. and may there- fore take its form from the cephalon (figure 61), or from the pygidium, as in the examples following (figures 62-65). During growth the new segments are added in front of the anal segment, so that after the number of abdominal seg- f f ] J[fijfc4 ments is complete the thorax is increased by the successive / r uL .Addition of what in earlier moults were pygidial segments. * /By this means the pygidium generally controls or determines ** J&8 cnarac ^ er f "the segments of the thorax. If the pleura /^xj^of the pygidium are extended into spiniform processes, the jjljy \ pleural ends of the segments are also spiniform; as in Lichas (figure 64), Ceraurus, Cheirurus (figure 62), Deiphon (figure 63), Acidaspis, Dindymene, etc. ORIGIN AND SIGNIFICANCE OF SPINES 79 Likewise, if the pleura or their distal ends are directed posteriorly nearly parallel to the axis, the mechanical neces- sities of motion require that the portions of the free segments pointing backward should be free, thus making the ends of 61 62 63 64 65 FIGURE 61. lUoenus (Octillanus) Hisingeri, Ordovician, Bohemia ; a trilobite ^ showing spiniform pleural extremities of first thoracic segment, corresponding to the genal spines of the cephalon. X f- (After Barrande. 4 ) /?(*, 0^ /^ FIGURE 62. Cheirurus msignis, Silurian, Bohemia ; pygidium and six. thoracic segments. X f. (After Barrande.) FIGURE 63. Deiphon Forbesi, Silurian, Bohemia; entire specimen; show- f^fT^ ing spiniform pleura of segments corresponding in direction to those of the (/ pygidium. (After Barrande.) FIGURE 64. Lichas scabra, Silurian, Bohemia; pygidium, with three thoracic segments ; showing spiniform ends of pleura. X f (After Barrande.) FIGURE 65. Paradoxides spinosus, Cambrian, Bohemia.; pygidium and six ^ j Jfr free segments. X/f- {After Barrande.) ^/>^ ju&JC*^ji%^ ^^Ski^^J ^^^^aJ^ff^ CS--#*&* ie tnoracic pleura generally appear as retrally curved spini- form extensions. Extreme examples of retrally directed pleura accompanied by small pygidia are shown in Para- doxides (figure 65), Holmia, Olenellus, Elliptocephala, etc. 80 STUDIES IN EVOLUTION Genera having the ends only of the pleura directed backward are generally less inclined to form spiniform terminations. In contrast with these it is found that all the Trilobita having the pleura directed outward, and with entire pygidial margins, do not ordinarily develop long pleural spines; as Asaphus, Illcenus, Agnostus, Phacops, Calymmene, etc. The examples of the caterpillars of moths belonging to the Schizurse, described by Packard 64 as mimicking the serrations of the leaves upon which they feed, have previously been noticed in this essay, under the head of mimetic influences. The initial cause of the spines may possibly be explained as in part due to the mechanical conditions. During their early existence the larvae feed on the lower side of the leaves, and have no spines. Later they feed on the edges of the leaves, at the same time acquire dorsal spines. The conforma- ^ rf /tion of the animal to the serrated edge of the leaf would ^ produce corresponding elevations and depressions on the back. The location of these would be fairly constant from the habit of the animal of feeding chiefly between the denser leaf veins which determine and terminate the serrations. The raised parts of the animal would receive the greatest amount of stimuli, and at these points spines would naturally appear. The processes producing the spines noticed in this category /(IX) are classed with others under decrescence, for the reason that the growth is restrained or controlled by mechanical necessities. If the restraint were absent, it is probable that a more expansive growth would take place or that other structures would be correspondingly benefited. f X. Disuse. (C 3 , D 2 .) In causing the reduction or atrophy of an organ, the effects of disuse have generally been recognized by most observers. In this way the origin of many of the so-called " rudimentary organs " has been satisfactorily explained by Darwin 14 and others. Two classes of structures are evidently comprised within the common definition of rudimentary organs ; namely, nascent and vestigial organs. ORIGIN AND SIGNIFICANCE OF SPINES 81 Nascent structures indicate the beginnings or initial stages of organs, while vestigial structures are the remnants left after the functional suppression of organs. The suppression is usually caused by unfavorable conditions or by disuse, which produces either a retardation of growth or a retrogres- sive development. In both cases the results are similar. By retardation an organ is prevented or restrained from func- tional development and is therefore useless as a normal organ. By retrogression an organ gradually reverts to an initial type, loses its function, and becomes a vestigial structure. In most instances a change of food or habit or the substitution of a new and functionally higher structure causes the disuse of some organ which under previous conditions was of use to the animal. Nascent structures, or the beginnings of organs, are gen- erally made up of active tissues that only require stimulus and nutrition to perfect their function. On the other hand suppressed or vestigial structures are composed of compara- tively inert tissue, and are in consequence largely made of the mechanical elements of secretion of the organism. It may therefore be considered that true rudimentary or nascent organs are potentially active, and suppressed structures are inert. It is with the latter class, the inert, that a study of spine genesis by atrophy is chiefly concerned. The gradual loss of function through disuse, and the con- sequent loss of nutrition with the concomitant rapid decres- cence of active tissues, bring about a change in the ratio of active and inert structures. The progression of this process naturally results in the production of a structure having a maximum of inert or mechanical tissues and a minimum of active constituents. Moreover, it has already been shown that the axial elements are the most persistent, and therefore the last to disappear; also that the peripheral appendages and outgrowths of any organ first show the action of decres- cence. Evidently the conditions here described are favorable for the production of spines out of an organ primarily possess- ing distinct active functions. The axis of an organ gives 6 82 STUDIES IN EVOLUTION the necessary form, and the hard tissue the structure, so that the whole will conform to the definition of a spine given early in this paper; namely, a stiff, sharp-pointed process. The restraint of the environment was found to be one * (cause for decrescence of organs. Another, which is properly the subject matter of the present section, is disuse; and lastly, it will be seen how the deficiency of growth force may bring a similar suppression of structures. There is considerable difficulty in selecting particular examples which will conform clearly to the strict require- ments of these three categories. In a certain sense some of the examples of spines produced by decrescence may belong to more than one category. However, it does not prevent the acceptance of any one of the three as primary causes. Thus it may be urged that disuse has caused the atrophy of leaves into spines among many desert plants, or produced a similar reduction of the limbs in a Python. While this may be true from one point of view, yet the manifest unfavorableness of the environment in both seems to be a sufficient reason for making it the primary factor. On the other hand many parasites showing similar atrophies are not dependent upon a large number of active organs for their food and maintenance. After finding a host an abundance of food is at hand, and the environment may be considered a favorable one. All the organs, except those of nutrition and reproduction, then become more or less useless and dwindle away, leaving vestigial organs or disappearing altogether. Furthermore, a change of habit, as from climbing to flying, will necessarily cause the atrophy of some of the structures used for climbing and the hypertrophy of others for flying. Most of the examples illustrating the production of a spine through the atrophy of an organ by disuse are to be found in the legs and digits of animals. The process bears consider- able resemblance to the formation of spines on many plants by the suppression of leaves, branches, etc. They will be noticed here, although properly these vestigial structures among animals are more strictly of the nature of claws, or, at the most, spurs. ORIGIN AND SIGNIFICANCE OF SPINES 83 Many parasitic plants, especially among the Balanophorese, are reduced to a simple stem bearing the inflorescence. The leaves are represented by scales which are often spiniform, though seldom of sufficient stiffness to entitle them to be called spines. In desert plants, many of which have a simi- lar type of growth, the hardening of the mechanical tissues by the effects of drought has converted similar leaf structures into spines, while the parasitic plants are not normally sub- jected to such continuous dryness and extreme heat, and therefore the mechanical tissues seldom become hardened. Parasitic animals, especially among the Crustacea and insects, often show a reduction in the number of joints in the legs, and even in the number of limbs themselves. The terminal claws generally persist, and are sometimes longer than the rest of the leg ; as in the Itch-mite Sarcoptes Scabiei, and in the female of the parasitic copepod Lernceascus nema- toxys (figure 66). Among many aquatic Crustacea and limuloids, the special- ization and segregation of the* ambulatory and swimming appendages toward the head or anterior regions of the body have produced a corresponding suppression of appendages on or near the extremity of the abdomen. This statement of fact is the basis of the principle of cephalization of Dana, 12 who applies it especially to the Crustacea, as follows : " There is in general, with the rising grade, an abbreviation relatively of the abdomen, an abbreviation also of the cephalothorax and of the antennsB and other cephalic organs, and a compacting of the structure before and behind; a change in the abdomen from an organ of great size and power and chief reliance in locomotion, to one of diminutive size and no locomotive power." Audouin's law that among the Articulata one part]' is developed at the expense of another may be also noticed here as affording a further explanation of the suppression of the posterior appendages correlative with the greater develop- ment of the parts anterior to them. In a Crustacean using its tail for propulsion, as the Lobster {Homarus), the telson is broad and flat, and the adjacent segment has a similar 84 STUDIES IN EVOLUTION development of the appendages. In other forms, as the Horse-shoe Crab (Limulus) and the Phyllocarida, the tail is not used for propulsion, and at best serves chiefly as a rudder, while some of the legs on the anterior part of the abdomen or on the thorax are large and strong and are often provided with paddles. These groups, the limuloids and Phyllocarida, show a greater or less suppression of the last abdominal appendages, and in many genera the body termi- nates in a spiniform telson or tail spine. The process of suppression may or may not result in a spine. In the crabs the abbreviated abdomen is folded under the cephalothorax, and in Lepidurus and Pterygotus the telson is a scale or plate-like organ. For the most part, however, the abbrevia- tion of the abdomen and the suppression of its appendages have reduced the telson to a spine; as in Limulus (figure 67), Eurypterus, Stylonurus, and PrestwicTiia among limuloids, and Olenellus among the Trilobita. In addition to a telson spine, the Phyllocarida have two lateral spiniform cercopods, the three spines together constituting the post-abdomen ; as in Ceratiocaris, Echinocaris (figure 68), Mesothyra, etc. Although the last abdominal segments of the Horse-shoe Crab have lost their appendages and show evidences of suppression, yet the tail spine is a large and useful organ, for it is of just the proper length to enable the animal to right itself after being overturned, which it is unable to do with its feet alone. The process of natural selection has doubtless in this way contributed to the development and retention of the long spine. This use cannot be ascribed to the tail spines of the Phyllocarida, though they evidently were important aids in directing movement, and also offered some degree of protection. The terminal claws on the phalanges of the wings of some birds are nearly all that remain of the external fingers or digits. In the Hoactzin of South America (Opisthocomus cristatus) the young bird has a thumb and index finger, both provided with claws, and climbs about much like a quad- ruped, using its feet, fingered wings, and beak. According ORIGIN AND SIGNIFICANCE OF SPINES 85 to Lucas, 43 a rapid change "takes place in the fore limb dur- ing the growth of the bird, by which the hand of the nestling, with its well-developed, well-clawed fingers, becomes the clawless wing of the old bird with its abortive outer finger." Similar claws or spurs occur on a number of other birds, some having functional wings, as in the example just de- scribed, and others having only vestiges of wings, as in the Wingless Bird of New Zealand (Apteryx, figure 69). 66 67 FIGURE 66. Female of LernceascuS nematoxys, a parasitic copepod ; showing suppression of limbs. Enlarged. (After Glaus.) FIGURE 67. Horse-shoe Crab (Limulus polyphemus) ; showing telson spine and abbreviated abdomen. Reduced. FIGURE 68. A Devonian phyllocarid (Echinocaris socialis) ; showing spiniform telson and cercopods. FIGURE 69. Wing of Apteryx australis, X i- (After Romanes.) FIGURE 70. Skeleton of right fore limb of the Jurassic Dinosaur Iguanodon bernissartensis ; showing partially suppressed first digit. X -fa. (After Dollo. 16 ) Another example may be taken from the Dinosaurian Rep- tiles. The Jurassic genus Iguanodon, from England and Belgium, belongs to a group (Ornithopoda) in which the number of functional digits varies from three to five in the manus, and from three to four in the hind foot. In this genus the hind foot had three functional toes, representing the second, third, and fourth of a normal pentadactyl foot. 86 STUDIES IN EVOLUTION The first is represented by a slender tarsal bone alone, while the fifth is completely suppressed. The manus, or fore foot, of this animal shows the second, third, fourth, and fifth digits of functional importance as digits, while the first is shortened and atrophied to the condition of a stout spur, standing out at right angles to the axis of the leg, as shown in figure 70. The fore legs of Iguanodon and others of the same order were short, and apparently used more for prehen- sion than locomotion, and in Iguanodon the suppression of the pollex, or thumb, into a spur doubtless provided the animal with a powerful weapon. Here is seen the suppression of a digit by loss of normal function, resulting in a protective structure of considerable value. XI. Intrinsic suppression of structures and functions. (Di.) The most obvious and direct relationship between an un- favorable environment and the suppression of structures to form spines was afforded by desert plants. In illustration of the intrinsic suppression of structures by deficiency of growth force, the vegetable kingdom again seems to offer the clearest evidences of a like relation between cause and effect. Instead, however, of taking an unfavorable environment, in the pres- ent instance a favorable environment must be assumed, and then a type which expresses in various ways its deficiency of growth force must be sought. In the desert plants it was found that no single family exclusively constituted the desert flora, but that a consider- able variety of types was present, and that some of these belonged to perfectly normal families commonly living under ordinary favorable conditions. Moreover, it was evident that there were certain types of form and habits of growth which were especially characteristic of plants living in desert or similar unfavorable regions. Therefore, to illustrate clearly intrinsic restraint or suppression of structures it will be necessary to take an environment which, in most re- spects, may be considered as favorable, and also a type of ORIGIN AND SIGNIFICANCE OF SPINES 87 plant life presenting evidences of a deficiency of growth force. The great groups of plants commonly known as brambles and climbing plants appear to meet most of the requirements. They abound in regions where the greatest luxuriance of vegetation is found, and are therefore chiefly characteristic of the tropics. Kerner 38 estimates that there are two thou- sand species of the true climbing plants in the torrid zone, and about two hundred in temperate regions. Tropical America has the largest number of species, the flora of Brazil and the Antilles being especially rich. In the sombre depths of the tropical forest the climbing plants, or "lianes," are not so abundant as in the open glades and along the edge of the forest, where the amount of light is greater and the conditions of existence are more favorable. As far as rich- ness of soil, amount of light, and degree of temperature are concerned, it must be admitted that their environment is as favorable as that of any of the associated plants having dif- ferent habits of growth. The difference between the strong and erect plants and the comparatively weak and climbing Tj^t/w^ forms is therefore not an extraneous one. It resides within .> A the plant structures themselves, and is an intrinsic character/ / or an expression of hereditary vital forces. The law of recapitulation demands that each individual during its development shall pass through an epitome or recapitulation of its ancestral history. In view of the fact that the young seedlings of climbing plants and brambles have the erect form and proportions of normal erect foliage stems, it is safe to infer that they have been derived from erect forms. Further evidence is afforded from the absence of climbing plants in the earlier terrestrial floras. It is obvious, therefore, that they have been developed out of erect forms by a process of degradation. The next striking feature to be noticed in climbing plants is their extreme slenderness, due to the general suppression of the plant body. They may attain lengths not reached by the highest trees, and yet the diameter of the trunk is but 88 STUDIES IN EVOLUTION a minute fraction of the length. The Climbing Palm, or Ratan, has stems of great length and tenuity. It has been stated that stems two hundred metres long have been ob- served having a uniform thickness of only from two to four centimetres. 38 The diameter of such a stem would be only one or two ten-thousandths of its length. The length of the internodes is another conspicuous character in climbing plants, and both this and the slenderness of the stems suggest the results obtained by growing ordinary plants in the dark, where the conditions are adverse to increased vitality. The transfer of function from one part of the plant to another, usually by a process of retrogression or degradation, is also very common. The first growth above the ground is a leafy stalk. Later, after the plant has attained a consider- able height, the lower portion puts out quantities of rootlets and loses its foliage. The rootlets may be mere dry threads or points of support for the stem; or, if they happen to encounter a crevice containing soil, they develop into true absorbent organs. In others the ends of the growing stems or any point on the stems, upon reaching the earth, may put out vigorous roots. These facts seem to show a lack of | positive differentiation throughout the plant, which admits of the substitution of a lower structure for a higher by the suppression of a higher function. Lastly, the general spininess of climbing plants and bram- bles is a well-known and conspicuous character. Kerner 38 says that "most, if not all, plants which weave into the thicket of other plants are equipped with barbed spines, prickles and bristles." These spiniform processes seem to fall naturally into two classes: first, those produced by the suppression of stipules, leaves, petioles, branches, etc. ; and second, those appearing as simple eruptions on the surface. The suppression of normal plant organs into special struc- tures, as tendrils and claspers, is extremely common, and, as already shown, this process, if carried far enough without complete suppression, will favor the production of a spini- form growth representing the axial elements of the organs. ORIGIN AND SIGNIFICANCE OF SPINES 89 The classes of organs thus affected are practically the same as those in desert plants, though varying somewhat in man- ner and degree. The consolidated type of plant body is naturally absent, for in this respect the diffuseness of climbing plants is quite antithetical. It does not seem nec- essary to give a long list of examples among the climbers, illustrating the suppression of organs into spines. Although 71 72 73 FIGURE 71. Leaf of Ratan (Dcemonorops hygrophilus). Reduced. (After Kerner.) FIGURE 72. Leaf of Ratan (Desmoncus polyacanthus). Reduced. (After Kerner.) FIGURE 73. Bramble (Rubus squarrosus). Reduced. (After Kerner.) apparently not of rare occurrence, spines produced in this way are not as common as among desert plants. Two figures of the pinnate leaves of Ratan are introduced here to show the suppression of a number of the terminal leaflets into spines (figures 71, 72). In Machcerium the stipules are converted into thorns. 62 A tropical Bignonia (B. argyro- violacea) has normal full-sized simple leaves, and suppressed leaves bearing two opposite leaflets on one stalk, and ending 90 STUDIES IN EVOLUTION "in a structure which divides into three limbs, with pointed hooked claws, and which is not unlike the foot of a bird of prey." 38 By far the greater number of spines on climbing plants are of the nature of prickles, and are not produced by the sup- pression of any particu]ar organ or organs, but appear usually without any very definite order. They represent outgrowths of the superficial layers, and hypertrophied plant hairs, or trichomes. The cause of these cortical eruptions is not clear, although they seem to be intimately connected with the gen- eral suppression of the plant body. They are therefore a secondary and not a direct result of suppression. Bailey 2 asserts that "probably the greater number of spinous pro- cesses will be found to be the residua following the contraction of the plant body." This connection is very apparent in the consideration of the suppression or contraction of various plant organs, but is less obvious when applied to the surface of the whole plant, though doubtless it is the true explana- tion. In continuation of this idea it may be suggested that the prickles represent aborted attempts on the part of the plant, through hereditary influences, to recover its former normal proportions. Or they may exhibit the action of the law of repetition acting in an organism where the initial cause of spine production is the intrinsic suppression of such structures as leaves, petioles, stipules, etc. The subsequent repetition of spines on other parts of the organism results in a series of homoplastic spines which are not homologous with those first formed. The prickles on climbing plants and brambles may often serve for purposes of protection (D 3 ), and enable the plant to cling to a support, but these utilitarian properties cannot be considered as an initial cause. Natural selection, also, prob- ably has fostered the development of certain types of spiny climbers and the production of adaptive characters. Never- theless, in studying these forms, it is necessary to revert to the original consideration of the localized suppression of normal plant structures, and to the general suppression of ORIGIN AND SIGNIFICANCE OF SPINES 91 the plant body as affording a more primary conception of the causes and modes of spine growth among climbing plants. In many cases of retrogressive series of animals there seems to be a close parallelism with some of the characters observed among the climbing plants. If the Ammonite fam- ily during the Cretaceous, or near the close of the Mesozoic, is taken as an example, it cannot be said that the environ- ment of these old-age or pathologic series is unfavorable in respect to food, temperature, etc., for with them are assor ciated many vigorous progressive series of other organisms. Neither can it be said that in many cases the animals perished on account of over-specialization^ though this was evidently the cause_of the extinction of a large number. The return to a condition of second cTTildhdod in old age cannot called a progressive specialization, since it clearly points to aT deficiency of growth force. Old-age types, or phylogerontic forms, among animals may show the same attenuation or suppression of the body as do climbing plants. Thus Baculites, considered by Hyatt as a typical phylogerontic type, has a very attenuate shell, and some species, after attaining a certain diameter, cease to increase in any direction except length. On account of being a chambered shell, it is manifest that the growth of the animal must have practically ceased, while its secretive ^~fj activities were continued and confined largely to lengthening^^ the shell. Other related genera of Cephalopoda show a simi-/& /** lar attenuation of the shell, evincing a stoppage of growth in the animal. Among the Mollusca it seems quite likely that attenuation of form often accompanies decreased growth power. The pathologic varieties of the Steinheim Planorbis, as described by Hyatt, 35 or of the recent Planorbis complanatus described by Fire*, 57 are further illustrations of this attenua- tion accompanying the uncoiling of the shell. The sedentary Magilus, immersed in its coral host, is also an example, for not only does the shell cease to increase in diameter, but the whole interior, except a small cavity at the end, is filled 92 STUDIES IN EVOLUTION with a solid deposit of lime. Similar examples could be multiplied indefinitely. Since, however, but few of them are spiniferous, their consideration does not properly come within the scope of the present discussion, though, as is well known, some of the attenuate forms often enlarge and contract periodically, such enlargements frequently leaving prominent laminae or nodes that are sometimes differentiated into spines. They suggest the observations on growth, senes- cence, and rejuvenation, made by Minot, 48 who showed that in guinea pigs from a very early age the increments of growth are in a steadily decreasing ratio to the increase of weight of the animal. This led to the general conclusion that the whole life of an individual is a process of senescence or growing old. Spines arising by a real pathologic or diseased condition of the individual can have little or no effect in producing a normal spiniferous variety or species. However, some note should be taken of them, especially as they may be con- genital, and thus appear through several generations. In the human species the peculiar skin disease known as ichthyosis sometimes produces spiniform excrescences, and the victims are commonly called "porcupine-men." The most cele- brated instance was the Lambert family. Haeckel 27 gives the following account of this family: "Edward Lambert, born in 1707, was remarkable for a most unusual and mon- strous formation of the skin. His whole body was covered with a horny substance, about an inch thick, which rose in the form of numerous thorn-shaped and scale-like processes, more than an inch long. This monstrous formation of the outer skin, or epidermis, was transmitted by Lambert to his sons and grandsons, but not to his granddaughters. The transmission in this instance remained in the male line, as is often the case." Other similar examples are cited by Gould and Pyle, 21 and the disease is described as "a morbid development of the papillae and thickening of the epidermic lamellae." ORIGIN AND SIGNIFICANCE OF SPINES 93 CATEGORIES OF INTERPRETATION Having thus far examined the factors governing the origin of spines, and found that they could be grouped into a number of distinct categories, it is now desirable to interpret these results, and endeavor to arrive at the real significance of the spinose condition. The two main generalizations which will be discussed are, first, that spinosity represents the limit of morphological variation, and, second, it indicates the decline or paracme of vitality. Spinosity a Limit to Variation. A number of data have already been given, leading to the belief that, on becoming spinose, organisms have reached a limit of morphological variation. They may continue to develop more and more differentiated and compound spines, but no new types evolve out of such a stock. The subject may be treated in two ways, both leading to the same conclusion. First, the stages and processes involved in the growth of a spine itself may be studied, and next the development of spines in the ontogenies and phylogenies of animals and plants may be examined. The growth of a spine has already been described, and it was shown that this type of growth may arise from speciali- zation of other ornamental features, such as nodes, ridges, and lamellae, and also from the decadence of leaves, legs, etc. These observations and numberless others which could be made, will be sufficient to show that almost any kind of superficial structure, as knobs, tubercles, ridges, laminae, reticulations, etc., has by differential growth been changed into spines; also, that organs of various kinds, as legs, branches, leaves, etc., have by atrophy been reduced to spines. In each case the parts in their development pass through the various intermediate stages, and clearly show that the spine is a result and not a mean. Moreover, none of these structures or organs are developed through the 94 STUDIES IN EVOLUTION contrary process ; namely, that of beginning with spines and passing through stages corresponding to laminae, ridges, tubercles, etc. The spine is the limit, and out of it no further structure is formed. It is necessary to make some mention here of the movable spines of Echinodermata, which appear to form an exception to the foregoing statements. There seems to be no doubt that the fixed and movable spines, the pedicellarise, the paxillse, and the spheridia, are homologous structures, and that all begin as spiniform skeletal outgrowths, which by subsequent growth and modification produce the structures mentioned (Agassiz 1 ). The echinoderm skeleton, including spines, etc., is deposited in the midst of living tissue, and in the case of the spines cannot be directly correlated with the spines of other classes of organisms, which are either very deficient in vitality or are dead structures as soon as com- pleted. After the movable spines of echinoderms are fully developed, the living portion is often confined to the base, and the shaft becomes simply a dead structure upon which encrusting organisms may find lodgment, a condition seldom occurring in the living spines. These finished spines never develop into anything else, and are the structures which conform to the present discussion. The embryonic condition of the spines and pedicellarise shows that they are really more internal than external structures, and therefore remain under the full control of the ordinary processes of growth, resorp- tion, and modification by living tissues. Furthermore, the movable spines are of such functional importance that no close homologies can be made with ordinary spines found in other classes of organisms. In tracing the ontogeny of a spinose form, it has been found (pp. 18-22) that each species at the beginning was plain and simple, and at some later period spines were grad- ually developed according to a definite sequence of stages. Usually after the maturity of the organism the spines reach their greatest perfection, and in old age there is first an ORIGIN AND SIGNIFICANCE OF SPINES 95 over-production or extravagant differentiation, followed by a decline of spinous growth, and ending in extreme senility with their total absence. There are abundant reasons for believing that the radicles of groups are undifferentiated and inornate, and whenever a class has had a long existence it has been by the continuance of such radical types or by the development of secondary or tertiary radicles, which, though differing in internal charac- ters, still retain a primitive simplicity in superficial features. The early stages of ontogeny of any form should agree with the radical stock, and, as already noted, these stages are simple. Hyatt 34 says on this point: "the evidence is very strong that there is a limit to the progressive complications which may take place in any type, beyond which it can only proceed by reversing the process and retrograding. At the same time, however, the evidence is equally strong that there are such things as types which remain comparatively simple, or do not progress to the same degree as others of their own group. Among Nautiloidea an4 Ammonoidea these are the radicle or generator types. No case has yet been found of a highly complicated, specialized type, with a long line of descendants traceable to it as the radicle, except the progres- sive ; and all our examples of radicles are taken from lower, simpler forms; and these radicle types are longer-lived, more persistent, and less changeable in time than their descendants." A few examples will now be taken from the life histories of large groups. In the Brachiopoda the order Protremata, containing most of the spinose forms, has 4 genera and 22 species in the Cambrian of America, 20 genera and 173 species in the Ordovician, and 30 genera in the Silurian. "Then began a steady decline, with extinction in the Car- boniferous of North America. In the Triassic of Europe this order is sparingly represented by small species, and is there essentially restricted to the family Thecidiidse, which continues to have living representatives in the Mediterranean 96 STUDIES IN EVOLUTION Sea" (Schuchert 64 ). The super-family Strophomenacea of this order is the longest lived, and excelled in amount of specific differentiation, there being 608 species in North America alone (Schuchert). In this super-family the early families and genera were without spines, it being only when Ohonetes is reached that the first spines are found in the order. In this genus they are along the hinge and seem to make up for the weak and obsolescent pedicle. Greater spine growth occurs in the genera Productella and Productus, where, in extreme cases, the surfaces of both valves are thickly studded. During the Carboniferous the spiny Pro- ducti attained their maximum both in number, length of spines, and in individual size, for here occur the largest species of all Brachiopoda. This was the climax. The Permian genera are chiefly degenerate forms (Aulosteges, Strophalosia), and with the close of the Paleozoic the family Productidse became extinct. The order Protremata, to which this family belongs, likewise underwent a rapid decline, and only two simple types continued on into the Mesozoic, while but one declining representative is living at the present time. Among the Ammonoidea the chief spiny forms are those occurring just before the final extinction of the group and representing the beginning of the decline of the order (Oriocenu, Toxoceras, Ancyloceras, Hamites, etc.). In the Dinosaurian Reptiles the great horned forms, Triceratops, Torosaurus^ etc., mark the extinction of the entire order. The great horned mammals of the Eocene, the Dinocerata, have left no descendants, and the giant Brontotheriidse, after undergoing various horn modifications through the Miocene, continued no further. It is not desirable, however, to convey the impression that the spines or horns are alone responsible for this wholesale extinction. It has been shown that they are undoubtedly often an expression of extreme specialization, and generally they represent the limits to which superficial structures may be differentiated. Although there may be other expressions for similar conditions, yet the presence of spines is one, if not ORIGIN AND SIGNIFICANCE OF SPINES 97 the most evident, marker of the attainment of these limits. The presence of a spine on an organ or part indicates the limit of progression or regression of that part or organ. If the spinose condition is general, or if it dominates important functions, it then indicates the limit of progression and regres- sion of the organism. Spinosity the Paracme of Vitality. The physiological interpretation of spinosity is a correlative of the morphological aspect of the same condition, and, as it was found that spinosity was a limit to morphological progress or regress, it will now be shown that it also indicates the par- acme or decline of physiological progress. Both inferences are drawn from the individual or ontogenetic standpoint, as well as from the racial or phylogenetic. In the spinose individual the decline of vitality has been studied by Geddes 20 in thorny plants. He concludes that they show a " gradual death from point backwards (i. e. ebbing vitality}" The requisite evidence is afforded in the experi- ence of gardeners who generally consider spiny plants as " always given to die back," or, as otherwise expressed, they " often prune themselves." It is difficult to adduce the same kind of evidence among animals, though there may be some degree of semblance between this self-pruning of spiniferous plants and the growth, death, and shedding of the antlers of the modern Deer. Stronger evidence of the relations of spinosity to the organism is afforded in the consideration of spines as consisting wholly of the mechanical tissues. They are more or less dead structures and are usually with- out special physiological function. Hence, in so far as the whole or a part of an organism is spinose, it represents the ratio between the mechanical and active tissues, or between the inert and living structures. Morris 49 correlates the mechanical and motor defences of animals and plants in a manner bearing upon this subject as follows : " If we examine the whole range of the animal kingdom, we find every phase of combination of mechanical 7 98 STUDIES IN EVOLUTION and motor defence, the motion growing more sluggish as the defensive armor grows more efficient. But in the whole king- dom motion persists as one of the defensive agencies. No animal exists without some power of motion, by whose aid it withdraws or otherwise escapes from danger." He also notes that the plant kingdom, with the exception of the minute, swimming forms, possesses no defensive motion, and that mechanical defence alone exists. Under mechanical defence are included thorns, spines, etc., together with chemical appli- ances ; as in plants with poisonous or disagreeable juices. These facts lead to the conclusion that, in proportion as animals are spinose or armored, they exhibit a vegetative type of structure, and have retrograded. It has been shown elsewhere in this article, that the great- est development of spinose organisms occurs just after the culmination of a group, and, as this period clearly represents the beginning of the decline of the vitality of the group, the spines are to be taken as the visible evidence of this decadence. A similar observation has been made by Packard, 54 who after passing in review the geological development of the Trilobita, Brachiopoda, and Ammonoidea, states that " these types, as is well known, had their period of rise, culmination, and decline, or extinction, and the more spiny, highly ornamented, abnor- mal, bizarre forms appeared at or about the time when the vitality of the type was apparently declining." ^ Furthermore, it is now commonly agreed that all groups V have been most plastic near their point of origin, or, in other words, that during their early history all the important or Y sf~ major types of structure have been developed. Their subse- quent history reveals the amount of minor differentiation and specialization they have undergone. Apparently, most of the early impulses of growth, whether from the environment or from vital forces, resulted in physiological changes producing fundamental variations in function and structure. The later influences of environment and growth force are expressed in peripheral differentiation, and show that the racial or earlier characters had become fixed, and that the later or specific ORIGIN AND SIGNIFICANCE OF SPINES 99 features were the chief variables. The stimuli which, during the early life history of a group, were expended in internal or physiological adjustments, later produce external differen- tiation, and in this differentiation spinosity is the limit. The presence of spines, therefore, indicates the fixity of the primary physiological characters, together with the consequent inability of the organism to change due to its decreasing vitality. Conclusion. Just as all the features of terrestrial topography are in- cluded between the limits of plains and mountains, and the mountains are considered as the limit of progressive ac- cidentatiou, so the spines of animals or the monticules and pinnacles of their surface may be considered as the limits of progressive differentiation. The primitive base level, or peneplain, becomes elevated, and by erosion is cut up into tablelands, mesas, and buttes, with intersecting valleys. The valleys are gradually deepened, and the country becomes rougher until a maximum is reached. Then follows a reduc- tion of the inequalities of the surface, and finally, in old age, the smooth, gently rounded outlines of geographic infancy again appear. So in organisms the smooth rounded embryo or larval form progressively acquires more and more pro- nounced and highly differentiated characters through vouth and maturity. In old age it blossoms out with a galaxy of spines, and with further decadence produces extravagant vagaries of spines, but in extreme senility comes the second childhood, with its simple growth and the last feeble infantile exhibit of vital power. The history of a group of animals is the same. The first species are small and unornamented. They increase in size, complexity, and diversity, until the culmination, when most of the spinose forms begin to appear. During the decline extravagant types are apt to develop, and if the end is not then reached, the group is continued in the small and un- specialized species which did not partake of the general tendency to spinous growth. 100 STUDIES IN EVOLUTION Lastly, it must be determined whether spines are really hereditable characters, and therefore can be used in studying the phylogenies of groups. No one has yet been able to show any type or set of characters which cannot be trans- mitted from parent to offspring. Hyatt 34 says : " Every- 74 Ontogeny stages. Ontogeny condition. Phylogeny stages. Phylogeny condition. Chro- nol- ogy- Old age or gerontic Paraplasis Phylogerontic Paracme 5 Adult or ephebic Metaplasis Phylephebic Acme 4 Immature or neanic Anaplasis Phyloneanic Epacme 3 Larval or nepionic Anaplasis Phylonepionic Epacme 2 Embryonic Anaplasis Phylembryonic Epacme 1 FIGURE 74. Diagram and table ; showing correlation of stages and condi- tions of development in the spinose individual, in its ancestry, and in time. thing is inherited or inheritable, so far as can be judged by the behavior of characteristics." Furthermore, in a review of animal life, extinct and living, no one can fail to be im- pressed with the fact that especially near the close of the life history of a group, or in a series of highly specialized forms, spinose characters are often considered as of supra- ORIGIN AND SIGNIFICANCE OF SPINES 101 varietal value, and are rated of specific, generic, and some- times of family rank, or even higher. They;hay^;^yefore{ acquired a fixed importance in these special groups, and ar and also in the " Grundziige" R6 of the same author. The order of arrangement, however, is very different. A great number of family divisions have been proposed, and undoubtedly many others will yet be made, but it is not within the province of this paper to deter- mine the precise value and limitations of the families. This would require discussions of priority and synonymy, and other- wise detract from the direct purpose of the writer; namely, to establish a basis for a natural classification, and in this way to apply what is currently known and accepted regard- ing the trilobites. Nevertheless, some notice must be taken of several families and genera which for various reasons do not appear here. The family Aglaspidae, including the genus Aglaspis Hall, proves to belong to the Merostomata and is therefore omitted. The family Bohemillidse has been shown by the writer 6 to have no foundation, because the type of the genus Bohemilla Barrande was based upon a mutilated specimen of ^glina. Several genera still commonly adopted are not here recog- nized in the lists under the families, since from the minute NATURAL CLASSIFICATION OF THE TRILOBITES 133 size of the individuals described and their immature char- acters they must be considered as the young of larger forms. Such are Conophrys Callaway, Cyphoniscus Salter, Holo- metopus Angelin, Isocolus Angelin, and Shumardia Billings. Triopus Barrande has been shown to be a chiton. FIGURE 75. Table of Geological Distribution of Trilobita. Much could be said against some of the recognized genera, but, as with the families, the writer has preferred in almost every case to adopt, for the present, what has been commonly accepted, and thus to avoid the entanglement of dates and synonyms which would be out of place in any general discus- sions. The type species of every genus is here made the central idea. It is taken as representing the genus more closely than any fortuitous assemblage of diverse species, which the next investigator may show belong to another or to several genera. Our ideas of a genus are naturally based mainly upon the species with which we are most familiar. Until forced to make authoritative comparative statements, it does not occur to one that the type of the genus under consideration may be quite different. An American stu- dent's conception of Homalonotus will probably be formed 134 STUDIES IN EVOLUTION largely upon the species commonly known as H. delphino- cephalus Green, from the Niagara, and H. DeKayi Green, from the Hamilton. The first time the type of the genus, H. Knighti Murchison, is seen he will be puzzled to place it. Similar examples could be multiplied indefinitely, and only show that the type must be taken as the ultimate unit of comparison. Diagnoses and Discussions of Orders and Families. Order A. HYPOPAKIA, nov. ord. (UTTO under, and irapfia cheek piece.) Free-cheeks forming a continuous marginal ventral plate of the cephalon, and in some forms also extending over the dorsal side at the genal angles. Suture ventral, marginal, or suhmar- ginal. Compound paired eyes absent; simple eyes may occur on each fixed-cheek, singly or in pairs. Including the families Agnostidse, Harpedidse, and Trinucleidse. This order includes the groups C and D, or the Ampycini and Agnostini of Salter, and also the family HarpedidaB of that author, which he included in the Asaphini. The special recognition of characters, however, between Salter's groups and the order here proposed is different. The presence of a part homologous with the free-cheeks of other trilobites has generally been more or less overlooked in the families of this order. In Trinucleus, Dionide, and Harpes the sutures have been correctly determined by Bar- rande. 3 Likewise, Angelin 2 gave the right structure in Ampyx, but in Agnostus this feature has escaped notice. The examination of extensive series of Agnostus, in the National Museum and in the Museum of Comparative Zool- ogy,* has proved that under favorable conditions of preserva- tion this genus shows a distinct plate, separated from the cranidium by a suture, and it can be compared only with the * In the former, through the courtesy of C. D. Walcott and C. Schuchert, and in the latter, of A. Agassiz and R. T. Jackson. NATURAL CLASSIFICATION OF THE TRILOBITES 185 free -cheeks in other trilobites, especially where they are con- tinuous around the front of the cephalon, as in Trinucleus and Ampyx. The presence of a hypostoma in Agnostus was also determined. Even in the higher genera of this order the suture is frequently unnoticed in descriptions, but it can be seen in all well-preserved specimens. In Trinucleus 29 and Harpes it follows the edge of the cephalon, and separates the dorsal from the ventral plate of the pitted limb. Since eye-spots occur on the fixed-cheeks in the young Trinucleus and adult Harpes, it is probable that this character is a primitive one in this order, and has been lost in Agnostus, Microdiscus, Ampyx, and Dionide. The ontogeny of Sao, Ptychoparia, TriartJirus, Dalmanites, etc., shows that the true eyes and free-cheeks are first devel- oped ventrally, appearing later at the margin, and then on the dorsal side of the cephalon. Therefore the Agnostidas, Trinucleida?, and Harpedidse have a very primitive head struc- ture, characteristic of the early larval forms of higher families. Other secondary features show ,that this order, though the most primitive in many respects, is more specialized than either of the others, except in their highest genera. The characters referred to are the glabella and pygidium. Very few species show the primitive segmentation of the glabella, it being usually smooth and inflated, and resembling in its specialization such higher genera as Proetus, Asaphus, and Lichas. The pygidium often fails to indicate its true num- ber of segments. Some Agnostus and Microdiscus show no segments either on the axis or limb of the pygidium. Trinu- cleus and others may have a many-annulated axis and fewer grooves on the pleural portions. The number of appendages corresponds to the axial divisions, as determined by the writer. 4 The multiplication of segments in the pygidium and their consequent crowding makes them quite rudimentary. Family I. AGNOSTIDJE Dalman. Small forms, having the cephalon and pygidium elongate, nearly equal, and similar in form and markings. Free-cheeks 136 STUDIES IN EVOLUTION ventral, continuous ; suture marginal or ventral. Eyes wanting. Thorax composed of from two to four segments, with grooved pleura. Cambrian and Ordovician. Including the genera Agnostus Brongniart and Microdiscus Emmons. The genera in this family are primitive in their form and structure, as shown by their ventral free-cheeks, marginal or ventral suture, elongate cephalon, and large pygidium. Some species have spines at the genal angles, corresponding to the interocular spines of Holmia and young Elliptocephala, and not to the spiniform projections of the free-cheeks. From their abbreviated thorax and progressive loss of annula- tions on the glabella and axis of the pygidium they must also be considered as degraded. Microdiscus, the earlier genus, has three or four free segments, and in some species (M. spe- ciosus Ford) preserves the normal pentamerous glabella and annulated pygidial axis, while the later genus, Agnostus, has but two free segments, and has lost the annulations of both glabella and pygidium. Matthew 26 has described the pro- taspis stage of Microdiscus, which agrees with the similar stage of Ptychoparia and Sao. Fully a dozen generic names have been proposed for forms of the general type of Agnostus, but none of them has ever come into current use. Nine were first published by Corda, 15 but as Barrande 3 subsequently showed that one was based on an Orbicula, another on a poor specimen of ^ffiglina, and three others on a single species, this grouping soon fell into disuse. Moreover, Barrande was inclined to give no generic value to the form and lobation of the glabella, and therefore all the species were placed by him in the single genus Agnostus. At the present time more weight is given to the characters of the glabella and pygidium, as indicating generic differences in dorsal and ventral structure, so that further study may show the desirability of restoring such of Corda' s names as were founded upon natural groups of this family. NATURAL CLASSIFICATION OF THE TRILOBITES 137 Family II. HAKPEDID^E Barrande. Cephalon large, margined by a broad expansion or limb; glabella short and prominent. Free-cheeks ventral, continu- ous; suture marginal, following the outer edge of the limb. Paired simple eye-spots, or ocelli, single or double, at the distal ends of well-marked eye-lines on the fixed-cheeks, extending outward from the glabella. Thorax of from twenty-five to twenty-nine segments, with long grooved pleura. Pygidiurn (in Harpes) very small, composed of but three or four segments. Cambrian to Devonian. Including Harpes Goldfuss, Harpina Novak, and Harpides ? Beyrich. The genus Harpes presents considerable variation in the lobes of the glabella. H. ungula Steinberg shows the full number of five lobes, but in some species, as H. d'Orlig- nyianum Barrande, the structure is like Cyphaspis, with separate basal lobes. Arraphus Angelin was apparently based upon a specimen of Harpes denuded of the pitted border. Harpides Beyrich is imperfectly known, but seems to belong here. The ocular ridges and tubercles on the fixed-cheeks, the broad limb, the glabella, and the narrow weak thoracic segments are all in accord with Harpes, though in other features it has affinities with the Conocoryphidae. In many respects Harpes is one of the most interesting genera of trilobites, since it is so unlike other forms. The broad hippocrepian pitted limb of the cephalon has its counterpart in Trinudeus and Dionide, although not so well developed in these genera. The cephalon is also comparatively longer and larger, both features being decidedly larval. It is the only family known in which functional visual spots, or ocelli, are situated on the fixed-cheeks. The young Trinu- deus has similar eye-spots, or ocelli. The great number of free segments in the Harpedidse is another primitive char- acter, although the cephalon (in Harpes) still remains larger than the thorax and pygidium. 138 STUDIES IN EVOLUTION Family III. TRINUCLEID^: Barrande. Cephalon larger than the thorax or pygidium; genal angles produced into spines. Free-cheeks continuous, almost wholly ventral, carrying the genal spines; suture marginal or sub- marginal. Paired simple eyes, or ocelli, generally absent in adult forms ; compound eyes wanting. Segments of thorax five or six in number, with grooved pleura. Pygidium triangular ; margin entire ; axis with a number of annulations ; limb grooved. Ordovician and Silurian. Including the genera and subgenera Trinucleus Lhwyd, Ampyx Dalman, Dionide Barrande, Endymionia ? Billings, Lonchodomus Angelin, Raphiophorus Angelin, and Salteria ? W. Thompson. The leading genera of this family form a tolerably homoge- neous group, although each has sometimes been recognized as characterizing a separate family. Trinucleus and Dionide have a broad pitted border, but this hardly seems of sufficient importance to remove them far from Ampyx, since the three genera agree in nearly all important structural details, as the extent and character of the free cheeks, the glabella, the number of free segments, and the character of the pygidium. Lonchodomus and Raphiophorus of Angelin are commonly admitted as sub-genera of Ampyx. Both Salteria W. Thompson and Endymionia Billings have been described as sub-genera of Dionide Barrande, though there is little positive evidence for this disposition of them. Until more perfect material representing these forms has been described, it will not be possible to decide satisfactorily upon their relationships or place in a classification. There- fore they are left with doubt in the present family. Order B. OPISTHOPAEIA, nov. ord. (omo-dev behind, and irapeia cheek piece.) Free-cheeks generally separate, always bearing the genal angles. Facial sutures extending forwards from the posterior part of the cephalon within the genal angles, and cutting the NATURAL CLASSIFICATION OF THE TRILOBITES 139 anterior margin separately, or rarely uniting in front of the glabella. Compound paired holochroal eyes on free-cheeks, and well developed in all but the most primitive families. Including the families Conocoryphidse, Olenidse, Asaphidse, Proetidse, Bronteidse, Lichadidee, and Acidaspidse. This order is nearly equivalent to group B, or the Asaphini of S alter, which included also the families Calymmenidee and Harpedidse, which belong elsewhere. The families which are here placed under this order lend themselves quite readily to an arrangement based upon the characters successively appearing in the ontogeny of any of the higher forms. Thus Sao, Ptychoparia, and other genera of the Olenidse have first a protaspis stage only comparable in the structure of the cephalon with the genera of the pre- ceding order, the Hypoparia. Therefore this stage does not enter into consideration in an arrangement of the families of the Opisthoparia. In the later stages, however, there is a direct agreement of structure with the lower genera of this order. The nepionic Sao, with two thoracic segments (Plate II, figure 2), has a head structure agreeing in essential features with that in Atops or Conocoryphe (Plate II, figures 14, 15). A later nepionic stage, with eight thoracic segments (Plate II, figure 3), agrees closely with adult Ptychoparia or Olenus (figures 16, 17). These facts clearly indicate that the family Conocoryphidse should be put at the base of this extensive order. Then, as Ptychoparia and Olenus are more primitive and simple genera than Sao, they, as typifying the family Olenidse, should govern its position, which accord- ingly would be next after the Conocoryphidse. In each case a family is considered as represented by its typical and most characteristic forms. It would be impossible to consider the advanced specialized genera of some families as representing their normal facies, for each one has undergone an indepen- dent evolution, and some characters have reached as great a degree of differentiation as will be found in much higher families. 140 STUDIES IN EVOLUTION It has been recognized that variations in the position of the eyes, the relative size of the free- and fixed-cheeks, and the degree of specialization of the glabella have a definite order in the ontogeny of any trilobite, and also that these characters have a greater taxonomic value than many others. Applying these principles in arranging the families which come under the Opisthoparia, we have the sequence as indi- cated above, beginning with the Conocoryphidae and followed by the Olenidse, Asaphidse, Proetidse, Bronteidse, Lichadidse, and Acidaspidse, in regular progression. See Plate II. figures 14-23. Family IV. CONOCORYPHIDJE Angelin. Free-cheeks very narrow, forming the lateral margins of the cephalon, and bearing the genal spines. Fixed-cheeks large, usually traversed by an eye-line extending from near the ante- rior end of the glabella. Facial sutures running from just within the genal angles, curving forward, and cutting the anterior lateral margins of the cephalon. Eyes rudimentary or absent. Thorax with from fourteen to seventeen segments. Pygidium small and of few segments. Cambrian. Including the genera and sub-genera Conocoryphe Corda (= Co- nocephalites Barrande), Aneucanthus Angelin, Atops Emmons, Avalonia Walcott, Bailiella, Matthew (= Salteria Walcott and Erinnys Salter), Bathynotus Hall, Carausia Hicks, Carmon Barrande, Ctenocephalus Corda, Dictyocephalites Bergeron, Eryx Angelin, Harttia Walcott, and Toxotis Wallerius. The genera coming under this family present a number of very primitive characters such as are shown only in the larval stages of higher forms. The free-cheeks are narrow and marginal, and can be compared with those in the nepionic stages of Sao and PtycJioparia. The eyes have not been detected, but the presence of an eye-line suggests their pos- sible existence. The variations of the glabella are very marked, and are as great as those which in higher forms attain some importance as family characteristics. In Toxotis, Carausia, and Aneucanthus the glabella expands in front, NATURAL CLASSIFICATION OF THE TRILOBITES 141 joining and forming part of the anterior margin, as in the glabella of the larval stages of Solenopleura, Liostracus, Ptychoparia, and Sao. Ctenocephalus and Eryx are slightly more advanced, as the glabella no longer marks the edge of the cephalon. In Atops,* Avalonia, Bathynotus, and Carmon the glabella is cylindrical, distinctly defined, and limited within the margin, and in Conocoryphe, Harttia, and Bailiella it narrows anteriorly, and only extends about two-thirds the length of the cephalon. Generally in this family the glabella displays its primitive pentamerous origin. In Bailiella and Oarausia two basal lobes are marked off from the fourth seg- ment by oblique furrows, as in Proetus and Cyphaspis. From a phylogenetic standpoint the family Conocoryphidse is at the base of this extensive order. As far as known, all the larval forms in the other families of the Opisthoparia agree in having the narrow marginal free-cheeks, bearing the genal angles. The eye-line is present in most of the adult Olenidse, and in the early stages of all as far as known, so that the general average of the characters in the Conoco- ryphidse represents the main larval features throughout the other families. They show, too, that although primitive in essential structure, differentiation through time has developed secondary features belonging to genera in higher families; as, for example, the basal glabellar lobes in Bailiella. Family V. OLENID^ Salter. Cephalon larger than the pygidiuin, usually wider than long; genal angles commonly produced into spines. Free-cheeks sepa- rate. Facial sutures extending forward from the posterior mar- gin of the cephalon along the eye-lobes, and either cutting the anterior margin separately or meeting on the median line. Eyes crescentic, reniform, or semi-circular, situated at the ends of eye- lines in all but highest genera. Trunk long, composed of from * Atops (type A. truineatus Eramons) seems to be a valid genus, and differs from Conocoryphe (type C. Sulzeri Schlotheim) in its glabellar characters, greater number of thoracic segments, and much smaller pygidium with fewer segments. 142 STUDIES IN EVOLUTION eight (?) to twenty-six free segments ; rarely capable of rolling up. Pygidium frequently small; margin entire or spinose. Principally Cambrian, but extending into the Ordovician. Including the genus Olenus Dalrnan as the type, and the fol- lowing genera and sub-genera, which should doubtless fall into several sub-family or even family groups: Acerocare Angelin, Acrocephalites Wallerius, Agraulus Corda, Angelina Salter, Anomocare Angelin, Anopolenus Salter, Asaphelina Bergeron, Bavarilla Barrande, Bergeronia Matthew, Boeckia Brogger, Cer atopy ge Corda, Chariocephalus Hall, Corynexochus Angelin, Crepicephalus Owen, Ctenopyge Linnarsson, Cyclognathus Lin- narsson, Dikelocephalus Owen (Centropleura Angelin), Dorypyge Dames, Ellipsocephalus Zenker, Elliptocephala Emmons, Euloma Angelin, Eurycare Angelin, Holmia Matthew, ffydrocephalus Barrande (==. young Paradoxides), Leptoplastus Angelin, Lios- tracus Angelin, Loganellus Devine, Menocephalus Owen, Mesona- cis Walcott, Micmacca Matthew, Neseuretus Hicks, Olenelloides Peach, Olenellus Hall, Olenoides Meek, Oryctocephalus Walcott, Palceopyge Salter, Parabolina Salter, Parabolinella Brogger, Paradoxides Brongniart, Peltura Angelin, Plutonides Hicks, Proceratopyge Wallerius, Protagraulus Matthew, Protolenus Matthew, Protopeltura Brogger, Protypus Walcott, Pteroce- phalia Koemer, Ptychaspis Hall, Ptychoparia Corda, Remopleu- rides Portlock, Sao Barrande, Schmidtia Marcou, Solenopleura Angelin, Sphceropthalmus Angelin, Telephus Barrande, Triar- thrella Hall, Triarthrus Green, and Zacanthoides Walcott. A complete study of this extensive family of trilobites would contribute much in the way of generic synonymy, and bring out the characters necessary for family determination and subdivision. This important work must be left for future investigation. So many genera have been described from separate cranidia or even pygidia as to make it impos- sible to deal with all of them in a systematic manner. The zeal to make the most out of the earliest known faunas has led many investigators to describe and recognize imperfect and poorly preserved material, and to establish genera upon very tenuous characters. Therefore, without a most inti- mate knowledge of all the forms, any grouping of the major- NATURAL CLASSIFICATION OF THE TRILOBITES 143 ity of the Cambrian genera into families or the limitations of the genera themselves must, as in the present instance, be taken tentatively and as necessarily incomplete. A number of genera have been already made the types of family divisions; as Paradoxides, Olenellus, Remopleurides, Ellipsoceplialus, Ptychoparia, etc. Some of them may be shown ultimately to possess characters of sufficient weight to be entitled to family distinction. A preliminary grouping of the best-known genera may be of some value here, and for the sake of convenience these divisions may be defined as sub-families. Four groups will be recognized, of which Paradoxides, Oryctocephalus, Olenus, and Dikelocephalus are taken as representative genera. I. Paradoxinse. Including Olenellus, Holmia, Mesonacis, Elliptocephala, Schmidtia, Ohnelloides, Paradoxides, Zacan- thoides, and Remopleurides. Most of the genera are distin- guished by their long narrow eyes, often extending more than half the length of the glabella, but more especially by the rudimentary character of the pygidium. In Olenellus the pygidium is a long telson-like spine. In Holmia, Meso- nacis, Elliptocephala, and Schmidtia it is reduced to a small plate without distinct segmental divisions. In Paradoxides, Zacanthoides, and Remopleurides the axis may show from one to five annulations, while the limb may carry two or three pairs of spines or may be entire. In Olenellus and Holmia true facial sutures have been denied by some authors, but in their place false sutures are recognized. They are, however, evidently real sutures in a condition of symphysis, which often occurs in Phacops, Proe'tus, Pliillipsia, etc. Otherwise these genera would violate the first principle of trilobite structure, in not having the compound eyes on the free- cheek pieces. Olenelloides is a very striking form, but its pygidium is unknown, and the head structure is obscure. The elongate cephalon is a decidedly larval feature, and the genal and interocular (?) spines strongly suggest its immature condition, and point to the possibility of its being the young of Olenellus or a related form. 144 STUDIES IN EVOLUTION There has been much discussion as to the synonymy and value of most of the names proposed as genera or sub-genera in this group. Paradoxides, Remopleurides, and Zacanthoides are about the only ones that have escaped severe criticism in recent years. Taking the type of each of the others, it is found that Elliptocephala (1844) was based on the species E. asaphoides Emmons, Olenellus (1862) on 0. Thompsoni Hall, Mesonacis (1885) on M. vermontana Hall sp., Holmia (1890) on R. Kjerulfi Linnarsson sp., Schmidtia (1890) on S. Mickwitzi Schmidt sp., and Olenelloides (1894) on 0. arma- tus Peach. Some of these names are generally recognized as sub- genera of Olenellus (Mesonacis, Holmia, Olenelloides), while others are considered as synonyms (Elliptocephala, Schmidtia). The early genera were described from very in- complete material, and therefore lacked sufficient diagnostic characters to define them clearly. At the present time nearly or quite entire specimens representing the type species are known, and it is possible to compare all the essential features with some degree of accuracy. The main characters offering the greatest variation are (1) the number of thoracic segments and (2) their specialization into groups, (3) the relative development of the third free segment, (4) the num- ber and position of the spine -bearing segments, (5) the form of the pygidium, (6) the presence or absence of interocular spines, and (7) the form of the cephalon. A simple varia- tion in any one of these would not necessarily imply more than a specific difference, but the genera here mentioned exhibit marked changes in all or nearly all of these charac- ters, and in any family should receive recognition. Olenellus, Mesonacis, and Elliptocephala are more closely related than the other forms, and probably have only a sub-generic value under Elliptocephala. In the first form with fourteen thoracic segments, the third is greatly enlarged and the fifteenth is the spiniform telson-like pygidium. In Mesonacis with twenty-six thoracic segments, the third is somewhat en- larged, and behind the narrow spine-bearing fifteenth seg- ment there are eleven others without spines, followed by the NATURAL CLASSIFICATION OF THE TRILOBITES 145 small plate-like pygidium. In Elliptocephala with eighteen thoracic segments, the cephalon is broader, the third segment is not enlarged except in the young, and the fourteenth to eighteenth segments are narrower and spine-bearing. II. Oryctocephalinae. Including Oryctocephalus, Cteno- pyge, Olenoides, and Parabolina, with large pygidia and all but the last one or two pleural elements continued into spines; also Eurycare, Angelina, Peltura^ and Protopeltura, with smaller and shorter pygidia and denticulations of the margins corresponding to the pleural divisions. III. Oleninee. Including Olenus, Agraulus, Liostracus, Acerocare, Ptychoparia, Solenopleura, PtycJiaspis, Leptoplas- tus, Loganellus, Sphceropthalmus, Parabolinella^ Boeckia, Pro- ceratopyge, Ceratopyge, Protypus, Ellipsocephalus, Sao, and Triarthrus. All these genera have small or medium-sized pygidia, with from two to eight annulations in the axis. Eyes medium to small, at the ends of distinct eye-lines in all but the latest genera, which preserve this character only during the young stages. Thoracic segments from eleven to eighteen. IV. Dikelocephalinse. Including Dikelocephalm, Asaphe- lina, and Crepicephalus. Eight or nine thoracic segments. Pygidium wide, with the posterior lateral portion often pro- duced into broad spine-like extensions. Dikelocephalus is in many ways related to Ogygia and Asaphus. Family VI. ASAPHID^B Emmrich. Cephalon and pygidium well developed; glabella often ob- scurely limited. Free-cheeks usually separate. Facial sutures extending forward from the posterior edge of the cephalon within the genal angles, and cutting the lateral or anterior margins, occasionally uniting in front of the glabella. Eyes smooth, well developed, sometimes of very large size, even occupying the entire surface of the free-cheeks. Thorax gener- ally composed of eight or ten segments, but varying from five to ten; capable of enrolment. Pygidium large, often with wide doublure. Cambrian, Ordovician, and Silurian. 10 146 STUDIES IN EVOLUTION The long list of genera in this family may be easily divided into two sections, which are often recognized as of family rank. I. ASAPHID^E. Including the genera and sub-genera Asaphus Brongniart (= Cryptonymus Eichwald), Asaphellus Callaway, Asaphiscus Meek, Barrandia McCoy, Basilicus Salter, Bathyurellus Billings, Bathyuriscus Meek, Bathyurus Billings, Bolbocephalus Whitfield, Brachyaspis Salter, Bron- teopsis W. Thompson, Dolichometopus Angelin, Geramphes Clarke, Holasaphus Matthew, Homalopecten Salter, Isotelus DeKay, Megalaspides Brogger, Megalaspis Angelin, Niobe Angelin, Ogygia Brongniart, Ogygiopsis Walcott, Phillipsi- nella Novak, Platypeltis Callaway, PtycTiopyge Angelin, and Stygina Salter. This is a tolerably homogeneous group, although some of the Cambrian forms have a sufficiently archaic expression to make them seem a little out of place with genera of so pronounced a family type as Asaphus, Niobe, Ptychopyge, Megalaspis, and Isotelus. The elements of the glabella are generally quite obscure, and even its limits cannot be clearly made out in late genera, as Stygina and Asaphus. The segmental nature of the gla- bella is clearly shown in Ogygia, Ogygiopsis, Homalopecten, Asaphellus, Bronteopsis, and Bathyuriscus. The elements of the pygidium are obscurely marked in Brachyaspis and Isotelus. Phillipsinella is a very small form, and probably the young of an Asaphus. Barrandia, Homalopecten, and Stygina serve as transition genera to the Illsenidse. II. ILL^ENHXE. Including the genera and sub-genera Ulcenus Dalman, ^glina Barrande (= Oyclopyge Corda), Buwastus Murchison, Dysplanus Burmeister, Ectillcenus Salter, Holocephalina Salter, Hydrolenus Salter, Illcenopsis Salter, Hlcenurus Hall, Nileus Dalman, Octillcenus Salter, Panderia Volborth, Psilocephalus Salter, Symphysurus Gold- fuss, and Thaleops Conrad. The Illsenidse form a much more compact group than the NATURAL CLASSIFICATION OF THE TRILOBITES 147 preceding, characterized by having a rostral plate and by the very tumid form of the large cephalon and the obscure or obsolete boundaries of the glabella and occipital lobe. The pygidium often closely resembles the cephalon in size and form, and the axis is frequently scarcely denned. Considerable variation is shown in the size, position, and direction of the visual surfaces. There is also a ratio be- tween the size of the fixed-cheeks and the eyes. In propor- tion as the fixed-cheeks are large, the eyes are small, and as the area of the fixed-cheeks diminishes from a widening of the axis of the animal, the eyes become larger. Thus, in Holocephalina, with extremely large fixed-cheeks and narrow axis, the eyes are quite small. In Hlcenopsis, Dysplanus, Panderia, and Octillcenus they are progressively larger, and in Illcenus, Bumastus, and Nileus, where the axis is wide and the fixed-cheeks are reduced, the eyes are relatively large. This variation reaches its limit in the species of jtEglina, where the axis is very wide and the fixed-cheeks are reduced to almost nothing, so that the glabella and eyes make up the entire dorsal surface of the cephalon. In ^glina princeps Barrande the eyes extend about half the length of the cepha- lon. The eyes of ^E. rediviva Barrande bound the whole length of the sides of the head, and in jffl. armata Barrande the coalesced free-cheek pieces are almost wholly converted into a visual area, so that there is a continuous eye around the sides and front of the cephalon. Variations in the position of the eyes are to be noted in nearly all the genera. In Ectillcenus and Psilocephalus they are in front of the middle of the length of the cephalon, and in Dysplanus, Illcenopsis, and Holocephalina they are near the posterior angles of the cranidium. Panderia has the eyes directed obliquely backward, and in Thaleops they are carried on conical extensions pointing outward. Family VII. PROETID^E Barrande. Cephalon about one-third of the whole animal; genal angles generally produced into spines; glabella tumid, with two lateral 148 STUDIES IN EVOLUTION basal lobes defined by oblique furrows in front of the neck seg- ment. Free-cheeks large, separate. Sutures extending from the posterior margin inward to the eyes, and then forward, cut- ting the anterior margins separately. Eyes prominent, often large. Thorax of from eight to twenty-two free segments, with grooved pleura. Pygidium usually of many segments; pleural and axial portions strongly grooved; margin entire or dentate. Ordovician to Permian. Including the genera and sub-genera Proetus Steininger, Arethusina Barrande, Brachymetopus McCoy, Celmus Angelin, Cordania Clarke, Crotalurus Volborth, Cyphaspis Burmeister, Dechenella Kayser, Griffithides Portlock, Phaetonella Novak, Phillipsia Portlock, Prionopeltis Corda, Pseudophillipsia Gem- mellaro, Schmidtella Tschernyschew, Tropidocoryphe Novak, and Xiphogomium Corda. The genera of this family readily fall into a series express- ing more or less closely the development and specialization of various characters. Arethusina is the only genus retaining the archaic eye-lines, and both on this account and for the comparatively forward position of the eyes (itself a nepionic character), together with the large number of thoracic seg- ments, it stands near the base of the series. The eyes gradually approach the axis, and move back- ward through the genera Tropidocoryphe, Cyphaspis, Proetus, Prionopeltis, Phillipsia, and Grriffithides. Concurrent with this variation, there is a reduction of the fixed-cheeks and exten- sion of the glabella. In Arethusina, Tropidocoryphe, Cor- dania, and Cyphaspis the fixed-cheeks are about the size of the free -cheeks, and occupy a large portion of the cranidium. They are more reduced in Proetus and Prionopeltis, and in Phillipsia and Grriffithides they form only a narrow border to the glabella. The lobation of the glabella varies greatly, and few species retain evidences of its original segmental nature. Some Proetus and Dechenella show this feature, but in many Phillipsia and Griffithides the elements cannot be made out. In Proetus there is often a small accessory lobe developed at the ends of the neck ring, which is only of NATURAL CLASSIFICATION OF THE TRILOBITES 149 interest as being homologous with similar lobes in many of the Lichadidse and Acidaspidse, where they often become very conspicuous. In all the Proetidse the oblique lobes of the fourth annulus of the glabella are also important in this connection, as here again is marked the inception of side axial lobes, which become prominent features in higher genera, indicating greater specialization of the organs and appendages of the head. Family VIII. BRONTEID^E Barrande. Dorsal shield broadly elliptical. Cephalon less than one- third the entire length; glabella rapidly expanding in front, with faint indications of lobes. Free-cheeks larger than the fixed-cheeks. Facial sutures extending from the posterior mar- gin just behind the eyes abruptly inward around the palpebral lobes, and then diverging and cutting the antero-lateral margins separately. Eyes crescentic. Thorax of ten segments, with ridged pleura. Pygidium longer than cephalon or thorax; axis very short, with radiating furrows* extending from it across the broad limb toward the margin; doublure very wide; margin generally entire. Ordovician to Devonian. Including the single genus Bronteus Goldfuss ( Goldius de Koninck). Many of the species of Bronteus (as B. angusticeps Barrande, B. palifer Beyrich) show a breaking up of the glabella into symmetrically disposed separate lobes, as in Conoliclias and Acidaspis. The frontal lobe is transverse and much larger than the others. Back of it may be simple grooves marking the elements (#. campanifer Beyrich), or there may be one or two circular or elliptical swellings on each side of the axis (B. angusticeps Barrande), or, in addition, the axial portion may consist of several lobes. The reduction of the axis of the pygidium and the expansion of the limb meet with their greatest expression in this genus. Lichas shows the decline of these characters, the pygidial limb becoming more or less deeply lobed, and finally the lobes are 150 STUDIES IN EVOLUTION represented by spines (Arges, Terataspis). Further progres- sion of these changes is shown in Acidaspis. Family IX. LICHADIDJE Barrande. Dorsal shield generally large and flat, with granulated test. Cephalon small, not more than one-fourth the entire length; genal angles spiniform. Free-cheeks separate; sutures extend- ing from the posterior margin obliquely inward to the eyes, and then almost directly forward, cutting the margin separately. Glabella broad, with a large, often tumid, central lobe and from one to three side lobes. Eyes not large. Thorax with nine or ten segments and grooved and falcate pleura. Pygidium large, flat, commonly with toothed or notched margin corresponding to the pleural grooves; doublure very broad. Ordovician to Devonian. Including the genera and sub-genera Lichas Dalman, Arcti- nurus Castelnau, Arges Goldfuss, Ceratolichas Hall and Clarke, Conolichas Dames, Dicranogmus Corda, Homolichas Schmidt, Hoplolichas Dames, Leiolichas Schmidt, Metopias Eichwald, Oncholichas Schmidt, Platymetopus Angelin, Terataspis Hall, Trochurus Beyrich, and Uralichas Delgado. Most of the forms of this family are above the average size of trilobites, and several species are among the largest of the class. They are all thin-shelled, and were loosely articu- lated, so that entire specimens are extremely rare. A great diversity is shown in the form and lobation of the glabella. In Lichas (sens, str.), Platymetopus, and Leiolichas the anterior lobe dominates and is continuous with the axis. In Hoplolichas and Homolichas the lateral lobes are strongly defined, and each is nearly equal in size to the central lobe. Dicranogmus, Oncholichas, Conolichas, Metopias, Arctinurus, and Arges show the lateral lobes divided transversely into two or three smaller ones. Lastly in Ceratolichas, and more especially in Terataspis, the central lobe becomes a promi- nent ovoid or globular extension. These variations evidently indicate differences in the relative development of the append- NATURAL CLASSIFICATION OF THE TRILOBITES 151 ages and organs of the head, and therefore are of consider- able morphological importance. The pygidium is composed of few distinct segments. The annulated portion of the axis is generally short, and the den- tations on the border of the limb, corresponding to the pleural grooves, range from two to four on each side. Leio- lichas is the only form which has an entire pygidial margin. Family X. ACIDASPIDJE Barrande. Dorsal shield spinose. Cephalon transversely semi-elliptical, quadrate, or trapezoidal ; genal angles spiniform. Glabella with one large median axial lobe and two or three lateral lobes. Free-cheeks large, separate. Sutures extending from within the genal angles abruptly inward to the eyes, and then forward, cutting the anterior margin each side of the glabella. Eyes small, often prominent. Thorax of eight to ten segments, with ridged pleura extended into hollow spines. Pygidium usually small, with spinous margin. Ordovician to Devonian. Including the genera and sub-genera Acidaspis Murchison, Ancyropyge Clarke, Ceratocephala Warder, Dicranurus Conrad, Odontopleura Emmrich, and Selenopeltis Corda. In this family and the Lichadidse is shown the highest expression of differentiation and specialization of the Opis- thoparia. The primitive pentamerous lobation of the axis of the cranidium is entirely obscured, and is only clearly seen in the protaspis and early nepionic stages. These two fam- ilies are very closely related, the chief differences being noticed in the size and character of the pygidium, and the ribbed or grooved pleura. The Lichades are generally much larger and flatter, but the smaller and highly spinose forms of Arges, Ceratolichas, and HoplolicJias approach quite near some of the Acidaspidse. It has been customary of late years to regard all the species of this family as belonging to the single genus Acidaspis, and to consider the various sub-divisions bearing separate names as of the value of sub-genera. Clarke H has shown that, on 152 STUDIES IN EVOLUTION the basis of priority, Oeratocephala is the first distinctive name ever applied to the group, and is therefore entitled to full generic recognition. He further recognizes Odonto- pleura, Acidaspis, Dicranurus, Selenopeltis, and Ancyropyge in the sub-ordinate position of sub-genera under Oeratocephala. Order C. PKOPAKIA, nov. ord. (TT/H) before, and irapeid cheek piece.) Free-cheeks not bearing the genal angles. Facial sutures extending from the lateral margins of the cephalon in front of the genal angles, inward and forward, cutting the anterior margin separately or uniting in front of the glabella. Com- pound paired eyes scarcely developed or sometimes absent in the most primitive family, well-developed and schizochroal in last family. Including the families Encrinuridse, Calymmenidae, Cheiru- ridae, and Phacopidae. Salter's first division, Phacopini, included the two families Phacopidse and Cheiruridse. The Calymmenidse were placed in his second division, the Asaphini. This is the only order of trilobites which apparently begins within the known Paleozoic, and, unlike the other orders, it had no pre-Cambrian existence. The earliest forms of the Proparia came at the close of the Cambrian, in the lower Ordovician. Its greatest generic differentiation was attained early. There is a rapid decline in the Silurian and Devo- nian, and only one or two genera extend to the beginning of the Carboniferous. In the Opisthoparia it was demonstrated that the Cono- coryphidse formed the natural base or most primitive family in the order, and was distinguished by the narrow marginal free-cheeks and the absence of well-developed eyes. It is of much interest and importance to be able to recognize, in the Proparia, a similar primitive family having characters in common with the former, but still clearly belonging to the NATURAL CLASSIFICATION OF THE TRILOBITES 153 higher order. Placoparia, Areia, and Dindymene, of the Encrinuridse, constitute a group of apparently blind trilo- bites, with narrow marginal free -cheeks, presenting in gen- eral the appearance of Atops, Conocoryphe, Ctenocephalus, etc., of the Conocoryphidse. The somewhat higher genera Cybele and Encrinurus have intermediate or transitional characters leading to the other families. The Cheiruridaa show a greater amount of differentiation and progressive and regressive evolution than any other in this order. Crotalo- cephalus arid Sphcerexochus seem to express the highest de- velopment, and Deiphon and Onycopyge show the effects of over-specialization, resulting in degeneration. The Calym- menidse, in their small eyes and narrow free-cheeks, have decided affinities with the lower genera. The same may be said of Trimerocephalus of the Phacopidse, though the other genera of this family possess large eyes, situated well back and close to the glabella. For these and other reasons, the family is placed at the end of the order, as expressing its highest development. Family XI. ENCRINURIDSE Linnarsson. Cephalon narrow, transverse. Fixed-cheeks very large. Free- cheeks long, narrow, separate, sometimes with a free plate between the anterior extremities. Sutures extending from in front of the genal angles obliquely forward, and cutting the anterior margin in front of the glabella. Eyes very small or absent. Thorax of from nine to twelve segments, with ridged pleura. Pygidium generally composed of many segments; limb with strong ribs usually less in number than the annulations of the axis. Ordovician and Silurian. Including the genera Encrinurus Emmrich (Cromus Barrande), Areia Barrande, Cybele Loven, Dindymene Corda, Placoparia Corda, and Prosopiscus Salter. The ConocoryphidaB were shown to be the radical of the order Opisthoparia, and for similar reasons the EncrinuridaB may now be taken as the primitive family of the Proparia. 154 STUDIES IN EVOLUTION The cephala of Areia and Placoparia have many resemblances to Conocoryphe, but the fixed-cheeks bear the genal angles and spines, while in the latter genus they are on the free- cheeks. In both families the free-cheeks are narrow and marginal, and the eyes are absent or rudimentary. Both these characters are decidedly larval. Other primitive and larval features belonging to the Encrinuridse are the eye- lines in Cybele and Encrinurus, the undefined and expanded termination of the glabella in Dindymene and Encrinurus, and the pentamerous head axis in all but Dindymene, in which the four anterior lobes or annulations are obsolete. In Encrinurus the eye-line in meeting and joining the anterior lobe of the glabella sometimes gives the appearance of an extra lobe, as in Ogygia and Paradoxides. Family XII. CALYMMENIDJS Brongniart. Cephalon somewhat wider than long. Fixed-cheeks large; genal angles rounded or produced into spines. Glabella nar- rowing anteriorly. Free-cheeks long, separate, usually with a free rostral plate between the anterior extremities. Sutures extending from just in front of the genal angles, converging anteriorly, and cutting the margins separately. Eyes small; visual surface seldom preserved. Thorax of thirteen segments, with grooved pleura. Pygidium of from six to fourteen seg- ments; axis tapering. Ordovician to Devonian. Including the genera and sub-genera Calymmene Brongniart, Brongniart ia Salter, Burmeisteria Salter, Calymmenella Ber- geron, Calymmenopsis Munier-Chalmas and Bergeron, Dipleura Green, Homalonotus Koenig, Koenigia (= Homalonotus) Salter, Pharostoma Corda, Plcesiacomia Corda, Ptychometopus Schmidt, and Trimerus Green. The genera of this family naturally cluster around the two leading ones, Calymmene and Homalonotus. Closely related to the first are Ptychometopus, Pharostoma, Calymmenopsis, and Calymmenella, all agreeing in having the glabella well defined and marked by furrows or indentations at the sides, corre- sponding to its segmental nature. NATURAL CLASSIFICATION OF THE TRILOBITES 155 The second group, including Brongniartia, Trimerus, Homalonotus (sens, str.), Plcesiacomia, Dipleura, and Bur- meisteria, agree in having a low, not sharply defined, quad- rate glabella, without distinct furrows or lobes. In general, the axis of the thorax and pygidium is much wider than in the first group, and the pygidium is more elongate and often pointed. Family XIII. CHEIRURID^E Salter. Glabella well defined. Free-cheeks small, sometimes much reduced. Sutures extending from in front of the genal angles inward to the eyes, and then obliquely forward, cutting the anterior margin in front and each side of the glabella. Eyes usually small. Thorax composed of from nine to eighteen seg- ments, generally eleven; pleura often extended into hollow spines. Pygidium small, with from three to five segments; pleural elements commonly produced into spines. Principally Ordovician and Silurian, but extending into the Devonian. Including the genera and sub-genera Cheirurus Beyrich, Actin- opeltis Corda, Amphion Pander, Anacheirurus Heed, Ceraurus Green, Crotalocephalus Salter, Cyrtometopus Angel in, Deiphon Barrande, Diaphanometopus Schmidt, Eccoptocheile Corda, Hem- isphcerocoryphe Reed, Nieszkowskia Schmidt, Onycopyge Wood- ward, Pseudosphcerexochus Schmidt, Sphcerexochus Beyrich, Sphcerocoryphe Angelin, Staurocephalus Barrande, and Youngia Lindstrom. As in other families, the most primitive genera are those in which the regular pentamerous lobation of the glabella is retained, with the eyes well forward, the free-cheeks narrow, and the fixed-cheeks ample. Diaphanometopus, Anacheiru- rus, Eccoptocheile, and Cyrtometopus agree in these respects, and therefore belong at the beginning of a phylogenetic list. Ceraurus and Nieszkowskia appear to branch off here, being characterized by the narrow transverse form of the cephalon and the great development of the two anterior pygidial pleura into hollow spines directed outward and backward. These features are simulated in Deiphon, in which, however, the 156 STUDIES IN EVOLUTION prominent glabella is without distinct lobes, and the large pleural extensions of the pygidium do not belong to the ante- rior segment. Its natural place is at the end of the series. F. Cowper Reed 31 has shown (in his memoir on the evolu- tion of Cheirurus and its sub-genera, not including the other genera of the family) that the direct line from Cyrtometopus passes through Cheirurus to Crotalocephalus. The genera Pseudosphcerexochus and Ampliion also have relations with these genera and should be placed here. There is next a group of forms with prominent globular glabellee, leading from Cheirurus to Sphcerocoryphe, and including Actinopeltis, Youngia, and Hemisphcerocoryphe. Staurocephalus should immediately follow these. Sphcerexochus seems to be related to Cheirurus and Actinopeltis. Like them it has two side lobes at the base of the glabella, and the anterior furrows are obsolescent, as in Actinopeltis and Youngia. Lastly come Onycopyge and Deiphon, with their globular glabellse with- out furrows, the spiniform fixed-cheeks, the thoracic and pygidial pleura, and the free-cheeks reduced to almost noth- ing, forming a small part of the doublure of the cephalon. The former genus has four spiniform pygidial pleura, two on each side, but in the latter two are reduced and the remain- ing pair is greatly enlarged. Family XIV. PHACOPIDJE Salter. Glabella tumid, widest in front. Free-cheeks continuous, united anteriorly. Suture extending from in front of the genal angles inward to the eyes, and then forward around the glabella. Eyes generally large, and always with distinct facets, schizo- cliroal. Thorax with eleven segments, with grooved pleura. Pygidium usually large and of many segments; limb ribbed; margin entire or dentate. Ordovician to Devonian. Including the genera and sub-genera PteopsEmmrich, Acaste Goldfuss, Chasmops McCoy, Coronura Hall, Corycephalus Hall and Clarke, Cryphceus Green, Dalmanites Emmrich (Hausman- nia Hall and Clarke), Homalops Bemele and Dames, Monorachos Schmidt, Odontocephalus Conrad, Pterygometpous Schmidt, Sym- phoria Clarke, and Trimerocephalus McCoy. NATURAL CLASSIFICATION OF THE TRILOBITES 157 The last family of trilobites comprises forms which are commonly believed to be the most highly organized of the class, and certain it is that a high degree of organization is manifested. Some of the characters may be considered as progressive, while others are larval or possessed chiefly by the most primitive families, and are therefore to be looked upon as regressive. Schizochroal eyes occur in no other family, and this feature is apparently indeterminate. The complete union of the free-cheeks, carrying the doublure of the sides and front of the cephalon, can be best homologized with similar structures in some of the lowest genera, and is a retention of the complete ocular segment. The glabella, though considerably enlarged anteriorly, does not attain the degree of specialization shown in Lichas and Acidaspis. Only Chasmops and related forms (Monorachos, Homalops, Symphoria, and Coronura) have separate or accessory lobes. The margin of the cephalon shows even greater diversity than in any other family. It may be plain (Phacops, Cry- phceus), notched (Corycephalus), denticulated (Odontocepha- lus), or extended in front as a spinose, spatulate, or dentate process (Dalmanites nasutus Conrad, D. tridens Hall, etc.). The pygidium has a range almost as great, though in this respect it is equalled in the Lichadidse, Acidaspidse, and some of the Olenidse. In America the section typified by Dalmanites culminated during the lower Devonian. Not only are the largest forms found here (Coronura diurus Green, C. myrmecophorus Green, D. tridens Hall, etc.), but also the most ornate and specialized; as Corycephalus, Odon- tocephalus, and Coronura. References. 1. Agassiz, L., 1873. Methods of Study in Natural History, eighth edition. 2. Angelin, N. P., 1854. Palaeontologia Scandinavica. Pt. I. Crustacea formationis transitionis. 3. Barrande, J., 1852, 1872. Systeme Silurien du centre de la Boheme. Part I. 1852 ; supplement, 1872. 158 STUDIES IN EVOLUTION 4. Beecher, C. E., 1895. Structure and Appendages of Trinucleus. Amer. Jour. Sci. (3), vol. xlix. 5. 1895. The Larval Stages of Trilobites. American Geologist, vol. xvi. 6. 1896. On the validity of the family Bohemillidse, Barrande. American Geologist, vol. xviii. 7. Bernard, H. M., 1892. The Apodidse. A Morphological Study. Nature Series. 8. 1894. The Systematic Position of the Trilobites. Quar. Jour. Geol. Soc. London, vol. 1. 9. 1895 Supplementary notes on the Systematic Position of the Trilobites. Quar. Jour. Geol. Soc. London, vol. li. 10. 1895. The Zoological Position of the Trilobites. Science Progress, vol. iv. 11. Brongniart, A., 1822. Histoire Naturelle des Crustace's fossiles. Trilobites. 12. Burmeister, H., 1843. Die Organisation der Trilobiten. 13. Chapman, E. J., 1889. Some remarks on the classification of the Trilobites as influenced by stratigraphic relations ; with outlines of a new grouping of these forms. Trans. Roy. Soc. Canada, vol. vii. 14. Clarke, J. M., 1892. Notes on the Genus Acidaspis. Report of the State Geologist, N. Y. State Mus., 44^ Ann. Rept. 15. Corda, A. J. C. [and J. Hawle], 1847. Prodrom einer Mono- graphic der bohmischen Trilobiten. Abhandl. bb'hm. Gesell. Wiss., Prag, vol. v. 16. Dalman, J. W., 1826. Om Palseaderna eller de sa kallade Trilobiterna. 17. Emmrich, H. F., 1839. De Trilobitis. Dissertation. 18. 1844. Zur Naturgeschichte der Trilobiten. 19. Gegenbaur, C., 1878. Elements of Comparative Anatomy, Eng- lish edition (Bell and Lankester). 20. Goldfuss, A., 1843. Systematische Uebersicht der Trilobiten und Beschreibung einiger neuen Arten derselben. Neues Jahrbuch fur Mineralogie, etc. 21. Hyatt, A., 1889. Genesis of the Arietidse. Mem. Mus. Comp. ZooL, vol. xvi. 22. Jackson, R. T., 1890. Phylogeny of the Pelecypoda. The Aviculidae and their Allies. Mem. Boston Soc. Nat. Hist., vol. iv. 23. Kingsley, J. S., 1894. The Classification of the Arthropoda. American Naturalist, vol. xxviii. 24. Lang, A., 1891. Text-book of Comparative Anatomy. English translation by H. M. and M. Bernard. NATURAL CLASSIFICATION OF THE TRILOBITES 159 25. McCoy, F., 1849. On the classification of some British fossil Crustacea, with notices of new forms in the university collec- tion at Cambridge. Ann. Mag. Nat. Hist. (2), vol. iv. 26. Matthew, G. F., 1896. Faunas of the Paradoxides Beds in Eastern North America. No. I. Trans. N. Y. Acad. Sci., vol. xv. 27. Milne-Edwards, A., 1873. Recherches anatomiques sur les Limules. Ann. Sci. Nat., t. xvii. 28. H., 1834-40. Histoire naturelle des Crustaces. 29. GEhlert, D.-P., 1895. Sur les Trinucleus de 1'ouest de la France. Bull. Soc. Geol. France (3), t. xxiii. 30. Quenstedt, F. A., 1837. Beitrage zur Kenntniss der Trilobiten. Archivfur Naturgesch., Bd. I. 31. Reed, F. R. Cowper, 1896. Notes on the Evolution of the Genus Cheirurus. Geological Magazine, vol. iii. 32. Salter, J. W., 1864. A Monograph of British Trilobites. Pt. I. Pal. Soc., London, vol. xvi. 33. Walcott, C. D., 1881. The Trilobite ; New and Old Evidence relating to its Organization. Bull. Mus. Comp. Zool , vol. viii. 34. Woodward, Henry, 1895. Some Points in the Life-history of the Crustacea in Early Palaeozoic Times. Anniversary Address of the President. Quar. Jour. Geol. Soc. London, vol. li. 35. Zittel, K. A., 1881-1885. Handjmch der Palseontologie, Bd. II. 36. 1895. Grundziige der Palseontologie. List of Genera. Page Acaste Goldfuss. 156 Acerocare Angelin. 142 Acidaspis Murchison. 151 Acrocephalites Wallerius. 142 Actinopeltis Cord a. 155 ^Eglina Barrande. 146 Aglaspis Hall. 132 Agnoslus Brongniart. 136 Agraulus Corda. 142 Amphion Pander. 155 Ampyx Dalman. 138 Anacheirurus Reed. 155 Ancyropyge Clarke. 151 Aneucanthus Angelin. 140 Angelina Salter. 142 Page Anomocare Angelin. 142 Anopolenus Salter. 142 Arctinurus Castelnau. 150 Areia Barrande. 153 Arethusina Barrande. 148 Arges Goldfuss. 150 Arraphus Angeliu. 137 Asaphelina Bergeron. 142 Asapliellus Callaway. 146 Asapliiscus Meek. 146 Asaphus Brongniart. 146 Atops Enimons. 140 Avalonia Walcott. 140 Bailiella Matthew. 140 Barrandia McCoy. 146 160 STUDIES IN EVOLUTION Basilicus Salter. 146 Bathynotus Hall. 140 Bathyurellus Billings. 146 Batliyuriscus Meek. 146 Bathyurus Billings. 146 Bavaritta Barrande. 142 Bergeronia Matthew. 142 Boeckia Brogger. 142 Bohemilla Barrande. 132 Bolbocephalus Whitfield. 146 Brachyaspis Salter. 146 Bracliymetopus McCoy. 148 Brongniartia Salter. 154 Bronteopsis W. Thompson. 146 Bronteus Goldfuss. 149 Bumastus Murchison. 146 Burmeisteria Salter. 154 Calymmene Brongniart. 154 Calymmenella Bergeron. 154 Calymmenopsis Munier-Chal- mas and Bergeron. 154 Carausia Hicks. 140 Carman Barrande. 140 Celmus Angelin. 148 Centropleura Angelin. 142 Ceratocephala Warder. 151 Ceralolichas Hall and Clarke. 150 Ceratopyge Corda. 142 Ceraurus Green. 155 Chariocephalus Hall. 142 Chasmops McCoy. 156 Cheirurus Bey rich. 155 Conocephalites Barrande. 140 Conocoryphe Corda. 140 Conolichas Dames. 150 Conophrys Callaway. 133 Cordania Clarke. 148 Coronura Hall. 156 Corycephalus Hall and Clarke. 156 Corynexochus Angelin. 142 Page Crepicephalus Owen. 142 Cromus Barrande. 153 Crotalocephalus Salter. 155 Crotalurus Volborth. 148 Cryphceus Green. 156 Cryptonymus Eichwald. 146 Ctenocephalus Corda. 140 Ctenopyge Linnarsson. 142 Cybele Loven. 153 Cyclognathus Linnarsson. 142 Cyclopyge Corda. 146 Cyphaspis Burmeister. 148 Cyphoniscus Salter. 133 Cyrtometopus Angelin. 155 Dalmanites Emmrich. 156 Dechenella Kayser. 148 Deiphon Barrande. 155 Diaphanometopus Schmidt. 155 Dicranogmus Corda. 150 Dicranurus Conrad. 151 Dictyocephalites Bergeron. 140 Dikelocephalus Owen. 142 Dindymene Corda. 153 Dionide Barrande. 138 Dipleura Green. 154 Dolichometopus Angelin. 146 Dorypyge Dames. 142 Dysplanus Burmeister. 146 Eccoptocheile Corda. 155 Ectillcenus Salter. 146 Ellipsoceplialus Zenker. 142 Elliptocephala Emmons. 142 Encrinurus Emmrich. 153 Endymionia Billings. 138 Erinnys Salter. 140 Eryx Angelin. 140 Euloma Angelin. 142 Eurycare Angelin. 142 Gerasaphes Clarke. 146 Griffithides Portlock. 148 Goldius de Koninck. 149 Harpes Goldfuss. 137 NATURAL CLASSIFICATION OF THE TRILOBITES 161 Harpides Beyrich. Harpina Novak. Harttia Walcott. Hausmannia Hall and Clarke. JSemisphcerocoryphe Reed. Holasaphus Matthew. Holmia Matthew. Holocephalina S alter. Holometopus Angelin. Homalonotus Koenig. Homalopecten IS alter. Homalops Remele and Dames. Homolichas Schmidt. Hoplolichas Dames. Hydrocephalus Barrande. Hydrolenus Salter. Illcenopsis Salter. Illcenurus Hall. Illcenus Dalman. Isocolus Angelin. Isotelus De Kay. Koenigia Salter. Leiolichas Schmidt. Leptoplastus Angelin. Liclias Dalman. Liostracus Angelin. Loganellus Devine. Lonchodomus Angelin. Mer/alaspides Brogger. Megalaspis Angelin. Menocephalus Owen. Mesonads Walcott. Metopias Eichwald. Micmacca Matthew. Microdiscus Emmons. Monorachos Schmidt. Neseuretus Hicks. NieszkowsJcia Schmidt. Nileus Dalman. Niobe Angelin. Page Page 137 Octillcenus Salter. 146 137 Odontocephalus Con r ad . 156 140 Odontopleura Emmrich. 151 Ogygia Brongniart. 146 156 Ogygiopsis Walcott. 146 155 Olenelloides Peach. 142 146 Olenellus Hall. 142 142 Olenoides Meek. 142 146 Olenus Dalman. 142 133 Oncholiclias Schmidt. 150 154 Onycopyge Woodward. 155 146 Oryctoceplialus Walcott. 142 Palceopyge Salter. 142 156 Panderia Voiborth. 146 150 Parabolina Salter. 142 150 Parabolinella Brogger. 142 142 Paradoxides Brongniart. 142 146 Peltura Angelin. 142 146 Phacops Emmrich. 156 146 Phaetonella Novak. 148 146 Pharostoma Cord a. 154 133 Pliillipsia Portlock. 148 146 Phillipsinella Novak. 146 154 Placoparia Corda. 153 150 Plcesiacomia Corda. 154 142 Platymetopus Angelin. 150 150 Platypeltis Callaway. 146 142 Plutonides Hicks. 142 142 Pnonopeltis Corda. 148 138 Proceratopyge Wallerius. 142 146 Proetus Steininger. 148 146 Prosopiscus Salter. 153 142 Protagraulus Matthew. 142 142 Protolenus Matthew. 142 150 Protopeltura Brogger. 142 142 Protypus Walcott. 142 136 Pseudopliillipsia Gemmellaro. 156 148 142 PseudospJicerexochus Schmidt. 155 155 146 Psilocephalus Salter. 146 146 Pterocephalia Roemer. 142 11 162 STUDIES IN EVOLUTION Page Pterygometopus Schmidt. 156 Ptychaspis Hall. 142 Ptychometopus Schmidt. 154 Ptycltoparia Corda. 142 Ptychopyge Angelin. 146 Raphiophorus Angelin. 138 Remopleurides Portlock. 142 Salteria Walcott. 140 Salteria W. Thompson. 138 Sao Barrande. 142 Sclimidtella Tschernyschew. 148 Schmidtia Marcou. 142 Selenopeltis Corda. 151 Shumardia Billings. 133 Solenopleura Angelin. 142 Sphcerexochus Beyrich. 155 Sphcerocoryphe Angelin. 155 Sphceropthalmus Angelin. 142 Staurocephalus Barrande. 155 Page Stygina Salter. 146 /Symphoria Clarke. 156 Sympliysurus Goldfuss. 146 Telephus Barrande. 142 Terataspis Hall. 150 Thakops Conrad. 146 Toxotis Wallerius. 140 Triarthrella Hall. 142 Triarthrus Green. 142 Trimerocephalus McCoy. 156 Trimerus Green. 154 Trinudeus Lhwyd. 138 Triopus Barrande. 133 Trochurus Beyrich. 150 Tropidocoryphe Novak. 148 Uralichas Delgado. 150 Xiphogomium Corda. 148 Youngia Lindstrom. 155 Zacanthoides Walcott. 142 2. THE SYSTEMATIC POSITION OF THE TRILOBITES * As a preface to these remarks, it may be stated that there is no intention of indulging in a controversy regarding trilobite affinities. Professor Kingsley, as a biologist and authority on living arthropods, naturally approaches the subject from a standpoint nearly opposite to that of a trilobite investigator or paleontologist. The differences of opinion or interpreta- tion held by each are generally more apparent than real, and, as stated, depend mainly upon the point of view. Further, it cannot be expected that students of Lang, Glaus, and Lankester will agree as to the value and significance of a number of important characters, or upon certain theories which have been the natural outcome of such differences. In the study of trilobite morphology and classification I have made homologies and correlations from theories, opin- ions, and observations which seemed most current and in general favor in standard text-books. The chief purpose of the investigation was to work out the structure and develop- ment of the trilobite, and to apply the information to a classi- fication of the members of the group itself. The results have been recently published in the American Journal of Science (February and March, 1897). No attempt was made to revise the classification of the animal kingdom from the trilobite standpoint, nor even to determine the branches of arthropod phylogeny. The discussion of the systematic position of Limulus was carefully avoided, though this is usually consid- ered the chief end of any trilobite theorizing. The affinities * This paper was written to follow one by J. H. Kingsley, on " The Sys- tematic Position of the Trilobites," published in the American Geologist, XX, 38- 40, 1897. 164 STUDIES IN EVOLUTION of the trilobites were manifestly closer to the Entomostraca and Malacostraca than to other arthropods, and therefore com- parisons were drawn with these sub-classes of the Crustacea. In the following remarks only the main points of difference between the views held by Professor Kingsley and myself are dwelt upon : If the trilobites are true crustaceans, as conceded, it is then fair to expect a more or less close agreement between the lar- val forms of both. In my paper on " The Larval Stages of Trilobites" (American G-eologist, September, 1895) I endeav- ored to show this close agreement, and concluded that the protaspis stage of trilobites could be homologized with the nauplius larva of higher Crustacea. Professor Kingsley notes the following differences : (1) The differentiated median and pleural regions; (2) the segmented cephalic region; (3) the absence of a median eye : and (4) paired eyes. As to the first, I do not think the differentiation is much greater than in the nauplii of Apus, Cyclops, Lucifer, and others in which there are side regions. The pleural regions cannot be considered as highly specialized characters, since they are common to many groups, and each segment is con- sidered as primarily consisting of tergum, pleura, and sternum. (2 ) The segmentation of the protaspis is very feeble in the earliest stages, and is evidently emphasized from the fact that the fossils are viewed as opaque objects and exhibit strongly any inequalities of surface features, while living nauplii are studied as translucent objects. Furthermore, any such dif- ference cannot be real, since the nauplius shows its true seg- mented nature in its paired appendages. (3) The apparent absence of a median eye in the trilobite protaspis could be taken as of some value were it not that the fossils are not more than one millimetre in length, and even under the most favorable conditions could hardly be expected to show such small features as ocelli. Moreover, the median eye may have been marginal or ventral, and there- fore would not be seen in the fossil, which preserves only the dorsal crust. SYSTEMATIC POSITION OF THE TRILOBITES 165 (4) Paired eyes are not present, or at least not visible in the protaspis stages of primitive trilobites. They may through acceleration appear in the protaspis stages of later genera, as they do in the nauplius embryos of certain modern decapods. I do not believe that the nauplius has any great phyloge- netic significance, and have considered it " as a derived larva modified by adaptation " (I. c., p. 190), and as a " modified crus- tacean larva " (ibid., p. 191). It does not seem necessary to correlate the post-oral second pair of trilobite appendages with the mandibles of higher Crustacea. The second pair in the nauplius is also post-oral and manducatory, though they later develop into the antenna and are pie-oral. As to the cephalon of a primitive crustacean, I have merely accepted the conclusion approved by Glaus, as stated by Lang, in his reconstruction of the original crustacean, which is as follows : " The head segment was fused with the four subse- quent trunk segments to form a cephalic region" (Comparative Anatomy, p. 406). Similarly in regard to the interpretation of the biramous appendages, I have adopted the statements and conclusions of a large number of zoologists who consider the most primi- tive appendages as branched or consisting of a dorsal and a ventral member, and I have followed them in thus interpret- ing the trilobite appendages, which are clearly of this nature. 3. THE LARVAL STAGES OF TRILOBITES * (PLATES III-V) INTRODUCTION. IT is now generally known that the youngest stages of trilobites found as fossils are minute ovate or discoid bodies, not more than one millimetre in length, in which the head portion greatly predominates. Altogether they pre- sent very little likeness to the adult form, to which, however, they are traceable through a longer or shorter series of modi- fications. Since Barrande 2 first demonstrated the metamorphoses of trilobites, in 1849, similar observations have been made upon a number of different genera by Ford, 22 Walcott, 34 ' 35> 36 Mat- thew, 26 ' 27 > 28 Salter, 32 Callaway, 13 and the writer 4 ' 5 ' 7 . The general facts in the ontogeny have thus become well estab- lished, and the main features of the larval form are fairly well understood. Before the recognition of the progressive transformation undergone by trilobites in their development, it was the custom to apply a name to each variation in the number of thoracic segments and in other features of the test. The most notable example of this is seen in the trilobite now commonly known as Sao hirsuta Barrande. It was shown by Barrande 3 that Corda 17 had given no less than ten generic and eighteen specific names to different stages in the growth of this species alone. The changes taking place in the growth of an individual * American Geologist, XVI, 166-197, pis. viii-x, 1895. LARVAL STAGES OF TRILOBITES 167 are chiefly : the elongation of the body through the gradual addition of the free thoracic segments ; the translation of the eyes, when present; the modifications in the axis of the glabella ; the growth of the free-cheeks ; and the final assump- tion of the mature specific characters of pygidium and ornamentation. In the present paper the larval stages of several species are described and illustrated for the first time, and a review is undertaken of all the known early larval stages thus far described. This work would have no special interest in itself were it not for the fact that, with our present under- standing of trilobite morphology, it is possible to reach some conclusions of general importance which have a direct bearing on the significance and interpretation of several of the leading features of the trilobite carapace, and incidentally upon the structure and relations of the nauplius of the higher Crustacea. The Protcfspis. Barrande 3 recognized four orders of development in the trilobites, as follows : TYPES ( Head predominating, incomplete. \ I. < Thorax nothing or rudimentary. >- Sao hirsuta. ( Pygidium nothing. ) ( Head distinct, incomplete. ) II. } Thorax nothing. [ Trmculeus ornatus and ( Pygidium distinct, incomplete. ) a11 Agnostus. r Head complete. \ III. -! Thorax distinct, incomplete. > Arethusina Konineki. (. Pygidium distinct, incomplete. ) f Head complete. ^ IV. < Thorax complete. > Dalmanites Hausmanni. ( Pygidium distinct, incomplete. ) A study of these groups shows at once that they form a progressive series in which the first alone is primitive. 168 STUDIES IN EVOLUTION The others are more advanced stages of development, as shown by the larger size of the individuals, and their hav- ing characters which appear successively in the ontogeny of a species belonging to the first order of development. To attain the stage which is represented by actual speci- mens, they must have passed through earlier stages, which as yet have not been found. Furthermore, it is evident that Barrande did not consider the orders after the first as primitive, and characteristic of the genera cited, for, in some remarks under the third order, he says: 3 "II est tres- vraisemblable, que la plupart des Trilobites de cette sec- tion, si ce n'est tous, devront etre un jour transfe're's dans la premiere, par suite de la ddcouverte probable d'embryons sans segmens thoracique." The geological conditions necessary for the fossilization of the minute larval forms of trilobites are such that only in comparatively rare instances are any of the immature stages preserved. Larval specimens are doubtless often over- looked or neglected by collectors, but generally the sedi- ments are too coarse for the preservation of these small and delicate organisms. In certain horizons and rocks, how- ever, such remains are quite abundant, and complete onto- logical series may be obtained. Yet it is not strange that series of equal completeness have not been found in all Paleozoic horizons. The abbreviated or accelerated development of many of the higher Crustacea has resulted in pushing the typical free- swimming, larval nauplius so far forward in the ontogeny that this stage is either eliminated or passed through while the animal is still within the egg, so that when hatched it is much advanced. Although the trilobites show distinct evi- dence of accelerated development through the earlier inherit- ance of certain characters which will be taken up later, yet it is not believed that the normal series or periods of transfor- mation were to any degree disturbed, since both the simplest and most primitive genera whose ontogeny is known and the most highly specialized forms agree in having a common LARVAL STAGES OF TRILOBITES 169 early larval type. This would be expected from their great antiquity, their comparatively generalized and uniform struc- ture, and from the fact that no sessile, attached, parasitic, land, or freshwater species are known. These conditions, by introducing new elements into the ontogeny, would tend to modify or abbreviate it in various ways, especially among the higher genera. Before discussing any of the various philosophical and theoretical problems involved in an attempt to correlate the larval forms of Crustacea, a brief consideration of the known facts relating to the larvse of trilobites will be presented. Minute spherical or ovoid fossils associated with trilobites have been described as possible trilobite eggs, by Barrande 3 and Walcott, 32 but nothing is known, of course, of the embryonic stages of the animals themselves. The smallest and most primitive organisms that have been detected, and traced by means of series of specimens through successive changes into adult trilobites, are, as stated above, little discoid or ovate bodies not mqre than one millimetre in length, as shown on Plates III and IV. It is fair to assume that we have here a general exhibition of trilobite larval stages, since the ten species represented are from various geological horizons belonging to the Cambrian, Ordovician, and Silurian sediments, with Devonian types, and showing the simple as well as the highly specialized forms. All the facts in the ontogeny of trilobites point to one type of larval structure. This is even more noticeable than among recent Crustacea, in which the nauplius is considered as the characteristic larval form. It is desirable to give a name to this early larval type apparently so characteristic of all trilo- bites, and among different genera varying only in features of secondary importance. This stage may therefore be called the protaspis (TT^COTO? primus, acrTrls scutum). The principal characters of the protaspis are the following: Dorsal shield minute, varying in observed species from .4 to 1 mm. in length; circular or ovoid in form; axis distinct, more or less strongly annulated; head portion predominating; 170 STUDIES IN EVOLUTION glabella with five annulations ; abdominal portion usually less than one-third the whole length of the shield, axis with from one to several annulations ; pleural portion smooth or grooved ; eyes when present anterior, marginal, or sub-marginal ; free- cheeks when present very narrow, marginal. Several moults took place during this stage before the complete separation of the pygidium or the introduction of thoracic segments. When such moults are recognized they may be considered as early, middle, and late protaspis stages, and designated respectively as anaprotaspis, metaprotaspis, and paraprotaspis. They introduced various changes, such as the stronger annulation of the axis, the beginning of the free-cheeks, and the growth of the pygidial portion from the introduction of new appendages and segments, as indicated by additional grooves on the axis and pleura. Similar ecdyses occur during the nauplius stage of many living Crustacea before a decided transformation is brought about. Certain of these later stages have received a distinctive name, and are called the metanauplius. It is believed that the protaspis is homologous with the nauplius or metanauplius of the higher Crustacea. Most of the reasons for this belief will appear later in the present paper ; some which may be stated now are as follows : (1) The size of the protaspis does not differ greatly from that of many nauplii, and represents as large an animal as could be hatched from the bodies considered as the eggs of trilobites. (2) Some of the sediments carefully examined by the writer could preserve smaller larval trilobites were such originally present and provided with a chitinous test, as shown by the abundance of minute ostracodes, and the per- fection of detail in these and other fossils. (3) The protaspis can be shown to be structurally closely related to the nauplius, and in a more marked degree pos- sesses some characters required in the theoretical crustacean ancestor. LARVAL STAGES OF TRILOBITES 171 Review of Larval Stages of Trilobites. Matthew 27> 28 has carefully described several early larval (protaspis) stages of trilobites from the Cambrian rocks t)f New Brunswick, which are very simple and primitive, and will be noticed first. Solenopleura Robbi Hartt; Plate III, figure 1; from the Cambrian of New Brunswick; after Matthew. 27 This larva is very minute and circular in outline; the glabella is ob- scurely annulated and extends to the anterior margin, where it is expanded ; the neck ring is the only one well defined ; the abdominal portion is less than one-third the whole length, and is limited by a slight transverse furrow; no traces of eyes or free -cheeks discernible. Liostracus onangondianus Hartt; Plate III, figure 2; from the Cambrian of New Brunswick; after Matthew. 27 This form is similar to the preceding, though larger, and with the glabella more rapidly expanding in front. The neck segment is the only one which is distinct. It should be mentioned that most of the larval specimens here described and figured are preserved in fine shales and slates, as casts of the interior of the dorsal shield, so that some features are not as emphatic as on the exterior of the test. When well preserved, the axis always shows the typical five annulations on the cephalon. Ptyclioparia Linnarssoni Walcott; Plate III, figures 3 and 4 ; from the Cambrian of New Brunswick ; after Matthew. ^ The earliest stage is slightly more elongate than the pre- ceding forms. The axis is narrow, expanding in front and obscurely annulated, five annulations belonging to the ceph- alon, and one to the pygidium, which is very short and separated from the cephalon by a distinct groove. The second stage (figure 4) is decidedly more elongate; the axis is more distinctly annulated; the occipital pleura defined; and the pygidium is larger and has an additional segment. Ptyclioparia Kingi Meek ; Plate III, figures 5, 6, and 7 ; 172 STUDIES IN EVOLUTION from the Cambrian of Nevada and Utah. Figure 5 represents a cast of the protaspis, and shows a defined occipital ring, with the axis slightly expanded and undefined in front ; py- gidium truncate behind. Figure 6, which is referred to a later stage (metaprotaspis) of the same species, shows the inception of several characters that have not as yet appeared in the previous larvae. The axis is very strongly annulated ; the anterior lobe is nearly as long as the four posterior annulations of the cephalon, and on each side there is a furrow representing the eye-line of the adult ; the free-cheeks are present as narrow marginal plates, including the genal spines; the pygidium shows two segments separated by a furrow. An adult Ptychoparia Kingi is shown in figure 7, and may be taken as representing the sum of the changes passed through in the development of larvae like the preceding, belonging to the genera Solenopleura^ Liostracus, and Ptycho- paria. The introduction and growth of the segments of the thorax are perhaps the most marked changes, but other points of importance to be noted are: the comparatively smaller size of the cephalon and its transverse form ; the limitation and recession of the glabella, which is now rounded in front, and only extends about two-thirds the length of the cephalon; the growth of the eyes and free-cheeks at the expense of the fixed-cheeks ; the increased segmentation of the abdomen, shown in the axial and pleural grooves on the pygidium. Sao hirsuta Barrande ; Plate ITT, figures 8, 9, 10, and 11 ; from the Cambrian of Bohemia ; after Barrande. 3 The speci- mens of this species are preserved as casts, and several of the features are therefore somewhat subdued. The earliest or anaprotaspis stage, represented in figure 8, is quite as primi- tive in most respects as any of the preceding. It is circular in outline, the annulations of the axis are distinctly shown only in the neck segment and pygidial portion, and the eye- line is present. In figure 9 of the metaprotaspis quite an advance is seen in the development of the free-cheeks and LARVAL STAGES OF TRILOBITES 173 the more pronounced annulation of the glabella, together with pleural grooves from the neck segment and those of the pygidium. The next stage (figure 10) probably represents the close of the protaspis stage (paraprotaspis) and the inception of the nepionic condition, when the cephalon and pygidium are distinct and before the development of the free thoracic segments. In considering the changes necessarily passed through by these larvae previous to attaining their adult characters (fig- ure 11) the most notable, aside from increase in size and addition of the sixteen thoracic segments, are : the appearance and translation of the eyes pari passu with the growth of the free-cheeks ; the growth of the border in front of the glabella, which now narrows anteriorly, and terminates about one- third the length of the cephalon within the margin ; the less distinct annulation of the glabella; and the development of the spines and tubercles ornamenting the test. Triarthrus Becki Green; Plate III, figures 12, 13, and 14; from the Ordovician, Utica slate, near Rome, New York. A larval form of this species was figured by the writer 6 in 1893. At this time the eye-line was confused with the anterior an- nulation of the axis, making the cephalon appear to have six instead of five annulations. A recent examination ofa large number of specimens shows that five is the invariable number, as here represented. Two protaspidian stages of this- species have been noticed, differing chiefly in the size of the pygid- ium. Both agree in showing a strongly annulated axis, not expanded in front and terminating some distance within the margin. From the first annulation a slightly elevated ridge on each side indicates the eye-line, and extends to the mar- ginal eye-lobe. The adult form (figure 14) shows, in addition to several characters noted in the previous species, the nearly complete loss of the two anterior annulations of the glabella ; the disappearance of the eye-line; and the development of a row of nodes along the axis, from the neck segment to the proximal segment of the pygidium. Acidaspis tuberculata Conrad; Plate IV, figures 1, 2, and 174 STUDIES IN EVOLUTION 3 ; from the Lower Helderberg group, Albany county, New York. 4 Several of these remarkable larvae have been found perfectly silicified in a limestone from which they have been freed by etching. In general form they resemble the second lar- val stage of Sao (Plate III, figure 9), but the pygidium is shorter and the glabella does not expand and terminate in the ante- rior margin. No eye-line is present, but the eye-lobes may be seen a little within the margin. The glabella has the charac- teristic number of annulations ; margin provided with a row of denticles ; genal angles extended into spines ; pygidium with four spines. The adult condition (figure 3) shows that the eyes have moved inward and backward to near the neck segment. The glabella has lost its annulations and is broken up into a median lobe with two smaller ones on each side, while the neck ring is projected into a spine. The changes noted here are much more profound than in any of the preceding genera, since Acidaspis is one of the most highly specialized of trilo- bites in its glabellar structure and elaborate ornamentation. The protaspis, too, partakes of this specialization, and although the general form of the shield and the annulation of the axis are as primitive as in Triarthrus, yet the characteristic spi- nosity of the genus appears even at this early stage and is a marked instance of acceleration of development. Arges consanguineus Clarke ; Plate IV, figure 4 ; from the Lower Helderberg group, Albany county, New York. A single larval form of this type has been found, and at first was provisionally referred to Phaethonides* The recent publica- tion by Clarke 14 of Arges consanguineus from the same horizon and a comparison of the larva with the description and with considerable additional material, renders it now possible to determine definitely the relations of this interest- ing form. As the main details of structure in Acidaspis and Arges are so similar, the transformations undergone by the larva are much alike in each case. The young Arges likewise shows the same acceleration in the development of the spines and surface ornamentation, and the retention of the primitive LARVAL STAGES OF TRILOBITES 175 features of the glabella. The specimen seen in figure 4 rep- resents a late larval stage (paraprotaspis), as shown by the transverse form of the cephalon and the large size of the pygidium. Proetus parvimculus Hall ; Plate IV, figures 5, 6, and 7 ; Utica slate, near Rome, New York. Two larval stages of this species have been found. The younger (figure 5) is smooth, broadly ovate, .72 mm. long, and widest in front ; axis distinctly annulated, cylindrical on the cephalon, tapering on the pygidium ; eyes nearly transverse to the axis, very large and prominent, situated on the anterior margin, sepa- rated only by the axis. The specimen represented in figure 6 is in the paraprotaspis stage, and measures .96 mm. in length. It shows an advance over the other in its size, its larger p} 7 gid- ium with grooved pleura, and the beginning of the recession of the eyes. The adult of this small species is shown in outline enlarged two diameters, in figure 7. The principal changes from the larva which should be noticed are : the loss of the four ante- rior annulations of the glabella, the neck segment being the only one wholly defined, although the basal lobes represent remnants of the next anterior ; the translation of the eyes backward as far as the pleura of the neck segment, and the change from a transverse to a parallel position with respect to the axis. In the original description of this species, 23 no mention was made of fine undulating strise ornamenting the entire dorsal surface of the test, nor of the basal lobes of the glabella. Both these features are present in the type specimen, which is from Cincinnati, Ohio, as well as in all the specimens from the Utica slate, near Rome, New York. With these additional characters the species is very closely related to Proetus deco- rus Barrande. Dalmanites socialis Barrande ; Plate IV, figures 811 ; from the Ordovician of Bohemia; after Barrande. 3 A nearly complete series of the growth stages of this species is given by Barrande. The earliest, or anaprotaspis, stage found (figure 8) exhibits an 176 STUDIES IN EVOLUTION outline and axis similar to Acidaspis. The eyes are quite large and situated, as in the same stage of Proetus, transverse to the axis, on the anterior border. Genal angles present, but in this case not produced by the free-cheeks as in Sao and Ptychoparia ; glabella strongly annulated, increasing in diam- eter anteriorly, although not expanding at the frontal mar- gin as in Sao, etc. In the two following stages (figures 9, 10), the pygidium increases in size, and the pleura are defined. To reach maturity (figure 11), eleven segments are devel- oped in the thorax, the glabella becomes more prominently developed in front, but the five annulations are maintained. The eyes have travelled in and back as far as the third cepha- lic segment, and their longer axes have swung around into a position parallel with the axial line, as in Proetus. The py- gidium has added many new segments, and the extremity is prolonged into a spine. Before proceeding further in the discussion of the protaspis, it is necessary to notice a number of forms of young trilobites which have heretofore been referred to the embryonic and larval stages, but which are now believed to belong to stages later than the protaspis. Besides the truly elementary forms described by Barrande and already noticed (Sao hirsuta and Dalmanites socialis), there are others which he referred to his second, third, and fourth orders of development. 3 Among these Agnostus may be taken first. The youngest forms of Agnostus nudus and A. rex (figures 76, 77) measure respectively 2 and 1.3 mm. in length, and the adults 13 and 15 mm. The earliest stages of the genera shown on Plates III and IV measure less than 1 mm., while the adults are more than 25 mm., with the exception of Proetus parviusculus, which is seldom more than 10 mm. long, though this species has a protaspis .72 mm. in length. The cephalon and pygidium of the youngest known Agnostus are quite separate. and distinct, which is not the case with the typical protaspis stage. It therefore seems probable that on account of the comparatively large size and LARVAL STAGES OF TRILOBITES 177 advanced structure of the youngest stages observed, the ele- mentary forms of this genus are as yet unknown, and possibly the extreme tenuity of the test in the protaspis has prevented their preservation. In the same way the young of Trinu- cleus (figure 78) show a separate cephalon and pygidium, and the specimens are in a much more advanced stage of develop- ment than the protaspis of Proiitm, shown on Plate IV, figure 5. An evidence of age is furnished, also, in the trans- verse shape of the head, which, in typical elementary forms, is longer than wide instead of wider than long. 76 77 8 78 79 81 83 FIGURE 76. Aynostus nudus Beyrich. (After Barrande.) FIGURE 77. Agnostus rex Barrande. (After Barrande.) FIGURE 78. Trinucleus ornatus Sternberg. (After Barrande.) FIGURE 79. Hydrocephalus saturnoides Barrande. (After Barrande.) FIGURE 80. Hydrocephalus carens Barrande. (After Barrande.) FIGURE 81. Olenellus (Mesonacis) asaphoides Emmons ; Ford collection. (Original X 30.) FIGURE 82. Olenellus (Mesonacis) asaphoides Emmons. (After Ford.) FIGURE 83. Olenellus (Mesonacis) asaphoides Emmons. (After Walcott.) The youngest specimens of Arethusina KoninM, figured by Barrande, 3 are 2 mm. or upward in length, and have seven or more free thoracic segments, with the cephalon wider than long. The facts of ontogeny show that younger stages must be admitted in which the number of segments diminishes to nothing, continuing down to a form agreeing with the protaspis of other genera. 12 ITS STUDIES IN EVOLUTION It has already been suggested 4 that the species described by Barrande 3 under the generic name of Hydrocephalm are probably the young of Paradoxides. This conclusion receives further support from the undoubted young of Olendlus, a related genus, which in its immature stages bears a strong resemblance to Hydrocephalus. The youngest examples of the latter have a distinct pygidium, a well-developed cepha- lon, and large eye-lobes at the sides of the glabella, as in adult forms. Free-cheeks were evidently present, though not generally preserved. See figures 79 and 80. The young of Olenellus asaphoides, described and illus- trated by Ford 22 and Walcott, 35 ' 36 also present a number of features considerably in advance of a typical protaspis. The immature characters are mainly the large size of the cephalon and the distinct annulation of the axis. The post-protas- pidian characters are the distinct and separate pygidium, the adult position of the eyes, and the apparently well-developed free-cheeks. In figure 82, after Ford, 22 the outer pair of spines belongs to the free-cheeks, the other pair being formed by the pleural extensions of the glabella, which were called the interocular spines. See also figures 81 and 83. The young specimen of PtycJioparia monile Salter sp., figured and noticed by Callaway, 13 is 1.5 mm. in length, and agrees, as far as can be determined without seeing the origi- nal, with what is known of other species of the same genus. It probably belongs to a stage later than the protaspis. Matthew 26 has carefully described some small cephala of Ctenocephaliis (Hartella) Matthewi and Oonocoryphe (^Baili- dld) Baileyi, from the Cambrian of New Brunswick. The fact of their being separate cephala, transverse in form, and from 2 to 3 mm. in length, is sufficient to show that they do not represent the youngest stages of these species. The immature examples of Agnostus, Trinucleus, Aretku- sina, Paradoxides^ Olenellus^ Ctenocephalus, and Conocoryphe, here briefly noticed, are of great interest in a study of the ontogeny of the various species to which they pertain. In the present paper, however, it is intended chiefly to establish LARVAL STAGES OF TRILOBITES 179 the primary larval characters of the trilobites, and therefore only the earliest stages are considered. Under the genera just mentioned the writer has endeavored to show that as yet their ontogeny cannot be traced as far back as the stage which has been defined as the protaspis. Therefore any general notions of first larval forms must at present be based on the genera Solenopleura, Liostracus, Ptychoparia, Sao, Triarthrus, Acidaspis, Proetus, and Dalmanites. Analysis of Variations in Trilobite Larvce. After taking a general survey of the earliest known larval stages of trilobites figured on Plates III and IV, it is evident that an accurate and detailed description of any one would not apply to any other except in certain broad characters. To formulate a definition of the protaspis applicable to all, as has been done previously (pp. 169 and 170), it is necessary to neglect or eliminate some rather striking characters which should now be mentioned. A few, features thus omitted are considered as very primitive larval characters, while others are modifications introduced in higher or later genera through the operation of the law of earlier inheritance. From the best evidence now obtainable, the eyes have migrated from the ventral side, first forward to the margin and then backward over the cephalon to their adult position, thus agreeing with Bernard's conclusions. 12 Therefore the most primitive larvae should present no evidence of eyes on the dorsal shield, and naturally there would be no free- cheeks visible. Just such conditions are satisfied in the youngest larva of Ptychoparia, Solenopleura, and Liostracus, which are the most primitive genera whose protaspis is known. The eye-line is present in the later larval and adolescent stages of these genera, and persists to the adult condition. In Sao it has been pushed forward to the earliest protaspis, and is also found in the two known larval stages of Triarthrus. Sao retains the eye -line throughout life, but in Triarthrus the adult has no traces of it, and none of the 180 STUDIES IN EVOLUTION higher and later genera studied has an eye-line at any stage of development. Matthew has considered this feature as especially characteristic of most of the Cambrian genera, and now it is further shown to be a character first appearing in the later larval stages of certain genera (Ptyclioparia, etc. ), next in the larval stages ($20), then disappearing from adult stages (Triartlirus), and finally pushed out of the ontogeny altogether (Acidaspis, Dalmanites, etc.). The eyes are visi- ble on the margin of the dorsal shield after the paraprotaspis stage, later than the eye-line in Ptychoparia, Solenopleura, Liostracus, Sao, and Triarthrus ; but in the other genera through acceleration they are present in all the protaspis stages, and persist to the mature, or ephebic, condition, moving in from the margin to near the sides of the glabella. The changes in the glabella are equally important and interesting. Throughout the larval stages the axis of the cephalon is five-segmented or annulated, indicating the pres- ence of as many paired appendages on the ventral side. In its simplest and most primitive state it expands in front, joining and forming the anterior margin of the cephalon (larval PtycJioparia, Sao). During later growth it becomes rounded in front and terminates within the margin. In higher genera through acceleration it is rounded and well-defined in front even in the earliest larval stages, and often ends within the margin (larval Triarthrus, Acidaspis). From these common types of simple, pentamerous glabellaB, all the diverse forms among adult individuals of various genera have been derived, through changes affecting any or all of the lobes. The modifications usually take place in the anterior lobes first, and gradually involve the others, though rarely disturbing the neck segment which is the most persistent of all. Six lobes are occasionally found in the glabella3 of some species. They do not indicate an additional pair of limbs, for the extra lobe is produced (a) by division of the anterior lobe through the greater or less extent of the eye-line across the axis, as in Olenellus, Paradoxides and Ogygia ; or (6) by the LARVAL STAGES OF TRILOBITES 181 marked development of muscular fulcra, which are supposed to be connected with the hypostoma. The next structures not especially noticeable in all stages of the protaspis are the free-cheeks, which usually manifest themselves in the meta- or paraprotaspis stages, though some- times even later. Since they bear the visual areas of the eyes, their appearance on the dorsal shield is practically simultaneous with these organs; and before the eyes have travelled over the margin the free-cheeks must be wholly ventral in position. They are very narrow when first dis- cernible (Plate III, figures 6, 9, and 10), and in Ptychoparia, Sao, etc., include the genal angles, but in Dalmanites they extend only a short distance below the eyes. The remaining features of the protaspis which here require notice are the pleural furrows and the pygidium. The pleura from the anterior segments of the glabella are occasionally shown, as in the young of Olenellus (figure 81), but usually the pleura of the neck segment are the first and only ones to be distinguished on the cephalon, ,fche others being so inti- mately coalesced as to lose all traces of their individuality. This makes the cranidium, or head shield, exclusive of the free-cheeks, consist of the fused lateral extensions or pleura of the head segments, as already noticed by Bernard. 12 The possible pleural or segmental nature of the free-cheeks will be noticed later. The distinct pleura of the pygidium appear soon after the anaprotaspis stage, and in some genera (Sao, Dalmanites') are even more marked than in the adult state, much resembling separate segments. The growth of the pygidium is very considerable through the protaspis stages. At first it is less than one -third the length of the dorsal shield, but by the successive addition of segments, it soon becomes nearly one- half as long. In some genera it is completed before the appearance of the free thoracic segments, though usually new segments are added during the adolescence of the animal. A number of genera present adult characters, which agree closely with some of the larval features noticed in this 182 STUDIES IN EVOLUTION section, and are important in a phylogenetic study of the trilobites. The main features of the cephalon in the simple protaspis forms of Solenopleura, Liostracus, and Ptychoparia are retained to maturity in such genera as Carausia and Acontheus, which have the glabella expanded in front, join- ing and forming the anterior margin. They are also without eyes or eye-line. Otenocephalus retains the archaic glabella nearly to maturity, and likewise shows eye-lines and the beginnings of the free-cheeks (larval Sao). Conocoryphe and Ptychoparia are still further advanced in having the glabella rounded in front, and terminated within the margin (larva of Triarthrus). These facts and others of a similar nature show that there are characters appearing in the adults of later and higher genera, which successively make their appearance in the protaspis stage, sometimes to the exclusion or modifica- tion of structures present in the most primitive larvae. Thus the larvae of Dalmanites or Proetus, with their prominent eyes, and glabella distinctly terminated and rounded in front, have characters which do not appear in the larval stages of ancient genera, but which may appear in their adult stages. Evidently such modifications have been acquired by the action of the law of earlier inheritance or tachygenesis. Altogether it seems that we have represented on Plates III and IV a progressive series of first larval stages in exact correlation with adult forms, the latter also constituting a progressive series, structurally and geologically. A summary of the features added to the dorsal shield of the anaprotaspis stage of acceleration during the evolution of the class, from the simpler forms of Cambrian times to the later and more highly differentiated Dalmanites, Proetus, and Acidaspis, would include: the free-cheeks; the eyes; the more strongly lobed glabella, rounded in front ; the transient eye-line ; the genal angles ; and the ornaments of the test. These additions, as may be seen by reference to Plates III and IV, considerably complicate and modify the primitive protaspis, but, as previously mentioned, it does not lose any of its essential structures. Besides, it is possible to trace LARVAL STAGES OF TRILOBITES 183 the origin and significance of the acquired characters, and thus to assign to each its true value. Antiquity of the Trilobites. The superlative age of the trilobites has been generally recognized, and is too well known to require more than a passing notice. Even in the earliest Cambrian they bear evidence of great antiquity in their diversified form, their larval modifications, and their polymerous cephalon and caudal shield, all of which features show that trilobite phylogeny must reach far back into pre-Cambrian times. Not only are the smallest species (Agnostus) found in the Cambrian, but also many of the largest (Paradoxides). There is a great range of variation in the number of free thoracic segments, varying from two in Agnostus to twenty in Paradoxides. The pygidium likewise shows extreme vari- ation of from two to upward of ten ankylosed segments. The eyes may be absent as in Agnostus and Microdiscus, or very large as in Paradoxides, though both in this respect and in the number of somites, free or fused, the Cambrian genera are exceeded in later deposits. In ornamentation and spiniform processes the Cambrian species show consider- able development, though not as great as others since that time. However, the wide variation they do present in this particular indicates differentiation and specialization consider- ably removed from the beginning of the trilobite phylum. The acquisition of distinct larval stages could only have been reached through a long series of changes in ancestral forms. The composition of the cephalon and caudal shield indicates a derivation from some primitive form, probably annelidan, in which, through adaptation to special require- ments, certain polar segments became fused, forming very distinct terminal body regions. Furthermore, the trilobites are the only large division of the Arthropoda which has be- come extinct. The Merostomata and Phyllocarida culmi- nated a little later, though still represented by living species; 184 STUDIES IN EVOLUTION but all the other divisions apparently have continued to increase since their inception during Paleozoic time. The only known arthropod contemporaries of the trilobites in the Cambrian are the Merostomata, Ostracoda, Phyllopoda, and Phyllocarida, all of the higher forms apparently having developed since that time. A more graphic view of the geological range and distribution of the arthropods is repre- sented in the following table : 84 P-i ^ 3 -3 Cenozoic Mesozoic Carboniferous Devonian Silurian Ordovician Cambrian Pre-Cambrian FIGURE 84. Geological Range and Distribution of Arthropoda. Having thus far reviewed the features of the primitive pro- taspis and some of the characters it acquired through earlier inheritance, together with the comparative age of the differ- ent groups of arthropods, it must be conceded that, in inter- preting crustacean phylogeny from the facts of ontogeny, the trilobites, so far as they show structure, are entitled to first place. Moreover, since the appendages are quite fully known and from them the trilobite proves to be a most generalized and primitive crustacean, still greater reliance can be placed LARVAL STAGES OF TRILOBITES 185 on deductions based upon a study of this type. The recent discoveries of the antennse and the exact details of trilobite structure, together with the larval homologies here made and the concordance of trilobites with the theoretical original crustacean, leave almost no doubt as to their true crustacean affinities. Woodward, 37 from another point of view, reaches the same opinion by saying : " The trilobita, being certainly amongst the earliest forms of Crustacea with which we are acquainted, cannot be removed from that class without destroying its ancestral record." Restoration of the Protaspis. At first thought the attempt to reconstruct the ventral side of the trilobite protaspis may seem a little hazardous or pre- mature, but a careful consideration of all the data leads the writer to undertake this with some confidence. The genus Triarthrus is taken for the basis of this restora- tion, as it is to-day the best known of all the trilobites, and its ventral structure has been ascertained to a degree of per- fection of detail which compares favorably with many of the recent crustaceans. 6 ' 7i 8> 9 The writer has studied the structure of many adult and immature specimens, some of them not more than 5 mm. in length, so that fortunately the appendages are known at many stages of growth. Especially are the young and rudimentary limbs near the extremity of the pygidium in adolescent individuals of considerable morphological inter- est, for they agree closely with the phyllopodiform trunk appendages in the metanauplius of Apus, and protozoea of Euphausia, or, in a general way, with the still more rudi- mentary trunk limbs in the nauplius stages of these and other forms. It has been definitely ascertained that the cephalon in trilo- bites bears five pairs of jointed appendages or limbs. 9 In lar- val or immature specimens, and in adults in which the glabella retains its primitive structure, this number is indicated on the dorsal shield by the five lobes or annulations of the glabella, 186 STUDIES IN EVOLUTION including the neck ring. These may therefore be taken as representing, in so far, the original segmentation of the cepha- lon, and agree with what is generally accepted as the primitive structure in modern true Crustacea. The head portion of the protaspis clearly shows this pentasomitic structure, and evi- dently carried a corresponding number of paired limbs on the ventral side. It has also been demonstrated that the annula- tions on the axis of the pygidium correspond to the number of paired limbs beneath, exclusive, of course, of the anal seg- ment. Here, too, it is possible to tell from the pygidial por- tion of the protaspis the number of limbs present during life. The protaspis of Triarthrus, represented in Plate III, figure 13, on this basis had five pairs of limbs attached to the head portion, and two pairs to the pygidium. Next, as to the composition and form of these elementary protaspis limbs, it is safe to assume that the anterior pair, corresponding to the antennules, must be uniramous, since they are so during all the young and adult stages observed, and since this form is common to all nauplius stages of modern Crustacea, and is recognized as primitive and elementary for the class. There is apparently a greater similarity in the larval antennules than between any other appendages, and as Apus and Euphausia have these in a very generalized form, they are taken as types of the first pair of limbs of the trilo- bite protaspis, as shown in Plate V, figure 1 (J). It should be noted, too, that the antennules of the trilobites arise from the sides of the upper lip or hypos toma, as in the nauplius. The other head appendages are typically branched, though in many of the recent Crustacea they lose this character after the larval stages. Especially is this true of the third pair of limbs, which become modified into the mandibles. In trilo- bites the primitive biramous structure of the head limbs per- sists to adult stages, occurring also in limbs of all the posterior segments where they become more and more phyllopodiform. 8 In the restoration of the protaspis it seems only necessary to append this archaic type of limb to each segment, agreeing as it does in form and structure with the rudimentary limbs of LARVAL STAGES OF TRILOBITES 187 older stages and with the nauplius and metanauplius stages of Apus. It cannot be doubted that the protaspis had five pairs of limbs on the head portion and one or more on the pygidium, and although these are the main points necessary to prove the argument in the next section, on the nauplius, yet it seems perfectly warrantable and better for graphic purposes to attach the required number of elementary limbs to the ventral side of the protaspis, as represented in Plate V, figure 1. There are other organs and structural details occurring in the nauplius and in adult trilobites, which deserve recogni- tion in a restoration of the protaspis stage. First among these is the labrum, or upper lip. Nowhere is this plate so well developed and so striking a ventral feature as among the tri- lobites. There can be no hesitation, therefore, in accepting this as characteristic of the protaspis. The trilobites and most recent crustaceans have a metas- toma, or lower lip. This is already developed in the nauplius stage of some Crustacea, as EupJiausia and Peneus, and prob- ably represents an early larval character. It usually appears as a median plate divided into two small plates, or lappets, on each side of the median line, posterior to the mouth, and is thus represented in the restored protaspis. As it occurs on a segment bearing also a pair of legs and has no separate neu- romere, it cannot well be considered as representing a somite. An anal opening is found in most nauplii, especially in those of the non-parasitic Crustacea, and in those in which this stage is normal and free-swimming. The protaspis, as representing a free-swimming larval stage of trilobites, there- fore probably possessed an anal opening. The only character represented in the restoration which is accepted purely from analogy is the median unpaired eye. This organ is almost universally present in the nauplius, and is regarded as a very primitive character wherever found. The next and last structures to be noticed are the free- cheeks and the beginnings of the paired eyes, as shown in Plate V, figure 1 (#, oo). Their existence has already been 188 STUDIES IN EVOLUTION indicated in the descriptions and observations of the protaspis and its derived characters, and need not be repeated here. Apparently the nauplius presents nothing homologous, unless possibly the frontal sensory organs of Apus, Balanus, Peneus, etc., may be taken as such. The paired eyes and frontal sensory organs are close together and seem to have some inti- mate connection, for, as the paired eyes develop, the latter dwindle and disappeear. Likewise in the trilobites the free- cheeks bear the visual areas, and may be almost wholly con- verted into eyes as in ^iEglina (Qydopyge). The greater or less separation of the cerebral ganglia in the chaetopods and in some of the lower Crustacea leads to the idea that the free-cheeks in trilobites are the pleura of an occuliferous head segment, which otherwise is lost. If the hypostoma is homologous with the annelid prostomium, as urged by Bernard, 11 then the free-cheeks may be considered as representing the second procephalic segment, which is the number required on the supposition that each neuromere cor- responds to a somite. There is a separate neuromere to each mesodermic metamere posterior to the head, and from anal- ogy we should expect that each neuromere in the cephalon would represent an original segment, especially as it can be demonstrated that the head is composed of consolidated or fused segments (Kingsley 24 ). Having thus shown the probable ventral structure of the protaspis, we are prepared to make some general observations on the larval type of modern Crustacea known as the nau- plius. Before doing this it is well to emphasize again that there is very positive evidence, amounting virtually to cer- tainty, that the protaspis had five pairs of limbs attached to the cephalic portion, behind which was an abdominal portion containing the formative elements out of which all the pos- terior somites and appendages were developed. The Crustacean Nauplius. The name Nauplius was first used by O. F. Miiller 29 to designate a minute crustacean believed to represent an adult LARVAL STAGES OF TRILOBITES 189 animal. Afterwards it was found to be a larval stage of Cyclops, but because it agreed in structure with the larvae of many other Crustacea the name was retained for that type of larval form and is now in general use. Primarily it is supposed to represent the first free-swimming stage after the escape of the animal from the egg. However, many species are quite fully developed when leaving the egg, and undergo comparatively slight subsequent metamorphoses, and in these and other species there may be developed in the egg an em- bryo having some of the characters of the nauplius. There- fore the term is also applied to all cases where a certain assemblage of nauplian characters occurs in the development of any crustacean. Thus it may be considered as a stage of development not restricted to a definite period of ontogeny. The adult Apus possesses so many nauplian features, and in its development passes through such simple metamor- phoses, that it has been aptly considered by Bernard ll as a nauplius grown to maturity. Balfour 1 also states that the chief point of interest in the development of Apus " is the fact of the primitive Nauplius form becoming gradually converted without any special metamorphoses into the adult condi- tion. " * This form, together with the nauplii of other crus- taceans and the study of the larval and adult characters of the trilobites, ought to afford definite knowledge of the char- acters possessed by the ancestral forms of the Crustacea. Before further examining the nauplius it may be well to state the characters which, on the grounds of comparative anatomy and phylogeny, are believed to represent the primi- tive adult crustacean. It will be seen that in many respects the trilobite recalls this type, but, as already suggested, is removed some distance from the prototype, although in itself a most primitive crustacean. Lang 25 gives a very comprehen- sive description of the racial form, as follows : " The original Crustacean was an elongated animal, consisting of numerous * The adult Apus properly has five pairs of cephalic limbs. A sixth pair of appendages has been correlated as maxillipedes, though from their innervation they seem to be metastomic and homologous with the chilaria of Limulus. 190 STUDIES IN EVOLUTION and tolerably homonomous segments. The head segment was fused with the 4 subsequent trunk segments to form a cephalic region, and carried a median frontal eye, a pair of simple anterior antennae, a second pair of biramose antennae and 3 pairs of biramose oral limbs, which already served to some extent for taking food. From the posterior cephalic region proceeded an integumental fold which, as dorsal shield, covered a larger or smaller portion of the trunk. The trunk segments were each provided with one pair of biramose limbs. Besides the median eye there were 2 frontal sensory organs. The nervous system consisted of brain, oesophageal commissures and segmental ventral chord, with a double ganglion for each segment and pair of limbs. The heart was a long contractile dorsal vessel with numerous pairs of ostia segmentally arranged. In the racial form the sexes were separate, the male with a pair of testes, the female with a pair of ovaries, both with paired ducts emerging externally at the bases of a pair of trunk limbs. The excretory func- tion was carried on by at least 2 pairs of glands, the anterior pair (antennal glands) emerging at the base of the second pair of antennae, the posterior (shell glands) at the base of the second pair of maxillae. The mid-gut possibly had segmen- tally arranged diverticula (hepatic invaginations). " The characters ascribed to the typical nauplius have been selected mainly on the principle of general average. They do not satisfy the theoretical demands resulting from a com- parative morphological study, nor are they consistent with the accepted requirements of an ancestral type of the Crustacea. Glaus 16 urges that the nauplius is a modified or secondary larval form, and the writer now hopes to further substantiate this view, and partly to reconstruct the nauplius from inter- nal evidence and from its more primitive representative, the protaspis of the trilobites. The usual features attributed to the nauplius are: three pairs of appendages, afterwards forming two pairs of antennae and the mandibles ; the first pair is uniramous and sensory in function; the second and third pairs are biramous, swimming LARVAL STAGES OF TRILOBITES 191 appendages; body usually unsegmented; anteriorly there is a single median eye, and a large labrum, or upper lip; an alimentary canal bent anteriorly, and ending in an anus near the posterior end of the body; a dorsal shield; the second pair of antennae are innervated from a sub-cesophageal gan- glion. Frontal sense organs and a rudimentary metastoma are sometimes present. The trunk and abdominal regions are not generally differentiated. Balfour 1 remarks of the nauplius that: "In most instances it does not exactly conform to the above type, and the diver- gences are more considerable in the Phyllopods than in most other groups." This variation is indeed quite marked among nearly all the groups besides the phyllopods, and furnishes the facts for the conclusion that the hexapodous condition is not primitive. On Plate V are represented some of the leading types of nauplius structure, taken chiefly from the excellent compila- tion by Faxon. 20 Bearing in mind the typical and average characters of this larva, some of the variations will be briefly reviewed. The nauplius of Apus, represented in Plate V, figure 2, shows the rudiments of five trunk segments, which in a later stage (figure 3) develop phyllopodiform appendages belong- ing to the sixth, seventh, and eighth pairs of limbs. They are the anterior trunk appendages, and appear at a time when the fourth cephalic pair is a mere rudiment while the fifth is entirely undeveloped. The fourth and fifth pairs of head appendages evidently must have some existence, though undeveloped in the nauplius. The physical conditions of nauplius life probably do not require them, and they there- fore remain for a time quiescent or undeveloped. In figures 4, 5, 8, and 6, respectively, of Branchipus, Artemia, Leptodora, and Limnaida, the first pair of append- ages becomes progressively shortened, until, in the last, they almost disappear. Leptodora (figure 8) and Lepidurus (figure 7) also have rudimentary trunk segments and append- ages (y). Figures 9 and 10, of Daphnia and Moina (from 192 STUDIES IN EVOLUTION summer eggs), show how rudimentary the nauplius append- ages may become when this stage is passed within the egg. Even a more marked reduction is exhibited in the embryos of Palcemon and Astacus (figures 25 and 26). Cyclops is a very normal form, though even here in a second nauplius stage (figure 12) a fourth pair of limbs is developed. Examples have been cited showing the reduction and obso- lescence of the anterior antennae, or first pair of nauplius limbs, and some cases will now be cited in which the third pair also becomes reduced and rudimentary. Achtheres (figure 14) and Mysis (figure 22) afford instances of this variation. The former is of additional interest, as showing that the appendages from the fourth to the eighth may be developed, while the third remains quiescent, and that the second pair, typically biramous, is here unbranched. Simi- larly, in Mysis, Nebalia (figure 19), and especially in Cypris (figure 18), the nauplius limbs are simple. The embryo of Lucifer (figure 24) and a late nauplius stage of Euphausia (figure 21) are also of moment in showing the beginnings of the metastoma (mt) with the two maxillae and first maxillipedes. It appears from the foregoing facts that enough has been shown to prove the marked variations in the number and state of development of the nauplius appendages, and to reach the conclusion that potentially five pairs of cephalic appendages are present. The two posterior pairs are the ones usually not developed until after some of the trunk limbs appear. Very satisfactory explanations have been offered as to why the first three pairs have been selected by the larva, although it does not seem to have been recognized that the fourth and fifth have been more or less suppressed daring the evolution of the class. Lang ^ accounts for the three pairs of nauplian limbs by saying that: "In a young larva which, like the Nauplius, is hatched early from the egg, only a few of the organs most necessary for independent life and independent acquisition of food can be developed. The 3 most anterior pairs of limbs which serve for swimming LARVAL STAGES OF TRILOBITES 198 may be described as such most necessary organs. The third pair perhaps belongs to this category, because as mouth parts, generally provided with masticatory processes, they serve not only with the others for locomotion, but also for conducting food to the oral aperture." Another point in favor of the original pentamerous compo- sition of the cephalic portion of the nauplius or protonauplius is the dorsal shield which is present in many forms, and is considered (vide Bernard n ) as a dorsal fold of the fifth seg- ment. So that, in reviewing the nauplius structures, we find here and there evidences of the entire series of head segments. Now, since the protaspis fulfils the requirements by hav- ing five well-developed cephalic segments, and is besides the oldest crustacean larva known, it is believed that, in so far, at least, it represents the primitive ancestral larval form for the class. The nauplius, therefore, is to be considered as a derived larva modified by adaptation. Other variations in the characters of the nauplius occur, but as they have clearly originated (a) from the parasitic habits of the adult, (6) from embryonic conditions, or (c) from earlier inheritance, they need not enter into considera- tion here. Such, for example, are (a) the absence of an intestine in Sacculina, (5) the absence of the median eye in Daphnia and Moina^ and (c) the bivalve shell in Cypris. The larval stages of other, and especially later and higher groups of arthropods, offer more considerable differences and need not enter into this discussion, which is aimed chiefly to establish the genetic relationship between the protaspis of trilobites and the nauplius of recent Crustacea. Summary. Barrande first demonstrated the metamorphoses of trilo- bites in 1849, and recognized four orders of development, which are now shown to be stages of growth of a single larval form. 13 194 STUDIES IN EVOLUTION A common early larval form is recognized and called the protaspis. The protaspis has a dorsal shield, a cephalic portion com- posed of five fused segments and a pygidial portion consisting of the anal segment with one or more fused segments. The simplest protaspis stage is found in the Cambrian genera of trilobites. During later geological time it acquired additional characters by earlier inheritance and became modi- fied, though retaining its pentamerous glabella and small abdominal portion. Some of these acquired characters of the dorsal shield are the free-cheeks, the eyes, the eye-line, the genal angles, and the ornaments of the test. The free-cheeks and eyes moved to the dorsum from the ventrum. The history of the acquired characters is traced by means of comparisons between larval and adult trilobites, through Paleozoic time, and a progressive series of larval forms estab- lished in exact correlation with adult forms, which them- selves constitute a progressive series, chronologically and structurally. The antiquity of trilobites is indicated by their remains in the oldest Paleozoic rocks, and especially by the fact that in the early Cambrian they are already much specialized and differentiated in number of genera. The age of the trilobite or crustacean phylum is further shown from the distinct larval stages of trilobites and their having a cephalon and pygidium of consolidated segments. Since the trilobites are among the oldest and most general- ized of Crustacea, their ontogeny is of considerable impor- tance in interpreting crustacean phylogeny. The protaspis in its segmentation shows that the cephalon had five pairs of appendages as in the adult. The crustacean nauplius is shown to be homologous with the protaspis and to have potentially five cephalic segments bearing appendages, which should therefore be taken as char- acteristic of a protonauplius. The nauplius is a modified crustacean larva. The pro- LARVAL STAGES OF TRILOBITES 195 taspis more nearly represents the primitive ancestral larval form for the class, and approximates the protonauplius. References. 1. Balfour, F. M., 1885. A Treatise on Comparative Embryology, memorial edition. 2. Barrande, J., 1849. Sao hirsuta Barrande, ein Bruchstiick aus dem " Systeme silurien du centre de la Boheme." Neues Jahrbuch fur Mineralogie, etc. 3. 1852. Systeme silurien du centre de la Boheme, l er partie. 4. Beecher, C. E., 1893. Larval forms of Trilobites from the Lower Helderberg Group. Amer. Jour. Sci. (3), vol. xlvi. 5. 1893. A Larval Form of Triarthrus. Amer. Jour. Sci. (3), vol. xlvi. 6. 1893. On the Thoracic Legs of Triarthrus. Amer. Jour. Sci. (3), vol. xlvi. 7. 1894. On the Mode of Occurrence, and the Structure and Development of Triarthrus Becki. American Geologist, vol. xiii. 8. 1894. The Appendages of the Pygidium of Triarthrus. Amer. Jour. Sci. (3), vol. xlvii/ 9. 1895. Further Observations on the Ventral Structure of Triarthrus. American Geologist, vol. xv. 10. 1895. Structure and Appendages of Trinucleus. Amer. Jour. Sci. (3), vol. xlix. 11. Bernard, H. M., 1892. The Apodidse. A Morphological Study. Nature Series. 12. 1894. The Systematic Position of the Trilobites. Quar. Jour. Geol. Soc. London, vol. 1. 13. Callaway, C., 1877. On a new area of Upper Cambrian rocks in South Shropshire, with a description of a new fauna. Quar. Jour. Geol. Soc. London, vol. xxxiii. 14. Clarke, J. M., 1894. The Lower Silurian Trilobites of Minnesota. In advance of vol. iii, Pt. ii, Geol. and Nat. Hist. Surv. of Minn. 15. Glaus, C., 1876. Untersuchungen zur Erforschung der genealo- gischen Grundlage des Crustaceen-Systems. 16. 1885. Neue Beitrage zur Morphologic der Crustaceen. Arb. zu. Inst. Wien, vi. 17. Corda, A. J. C. [and I. Hawle], 1847. Prodrom einer Monographic der bomischen Trilobiten. Abhandl. bohm. Gesell. Wiss., Prag, vol. v. 18. Dohrn, Anton, 1870. Untersuchungen iiber Bau und Entwickelung der Arthropoden, Zeitsch. fur Wiss. Zool., Bd. XXI. 196 STUDIES IN EVOLUTION 19. Dohrn, Anton, 1870. Geschichte des Krebs-Stammes nach embryo- logischen, anatomischen und pakeontologischen Quellen. Jenaische Zeitsch., vol. vi. 20. Faxon, W., 1882. Crustacea. Selections from Enibryological Monographs. Mem. Mus. Comp. ZodL, vol. ix, No. 1. 21. Fernald, H. T., 1890. The Relationships of Arthropods. Studies from the Biological Laboratory, Johns Hopkins Univ., vol. iv, No. 7. 22. Ford, S. W., 1877. On Some Embryonic Forms of Trilobites. Amer. Jour. Sci. (3), vol. xiii. 23. Hall, James, 1860. New Species of Fossils from the Hudson- River Group of Ohio and other Western States. Appendix, Thirteenth Ann. Rept. N. Y. State Cabinet. 24. Kingsley, J. S., 1894. The Classification of the Arthropoda. American Naturalist, vol. xxviii. 25. Lang, Arnold, 1891. Text-Book of Comparative Anatomy. Eng- lish translation by H. M. and M. Bernard. 26. Matthew, G. F., 1884. Illustrations of the Fauna of the St. John Group continued : On the Conocoryphea, with further remarks on Paradoxides. Trans. Roy. Soc. Canada, vol. ii, section iv. 27. 1887. Illustrations of the Fauna of the St. John Group. No. IV. Part II. The Smaller Trilobites with Eyes (Ptychoparidse and Ellipsocephalidae). Trans. Roy. Soc. Canada, vol. v, section iv. 28. 1889. Sur le Developpement des premiers Trilobites. Ann. Soc. Roy. Malac. de Belgique. 29. Miiller, O. F., 1785. Entornostraca, seu Insecta testacea, quae in aquis Daniae et Norvegia reperit, etc. 30. Miiller, Fritz, 1864. Fiir Darwin. 31. Packard, A. S., Jr., 1883. A Monograph of North American Phyl- lopod Crustacea. Twelfth Ann. Rept. U. S. Geol. and Geol. Surv. 32. Salter, J. W., 1866. A Monograph of British Trilobites. Part III. Pal Soc., London, vol. xviii. 33. Walcott, C. D., 1877. Notes upon the Eggs of Trilobites. Pub- lished in advance of Thirty-Jirst Rept. N. Y. State Mus. Nat. Hist. 34. 1879. Fossils of the Utica Slate and Metamorphoses of Triarthrus Becki. Printed in advance of Trans. Albany Inst., vol. x. 35. 1886. Second Contribution to the Studies on the Cambrian Faunas of North America. Bull. U. S. Geol. Surv., No. 30. 36. 1890. The Fauna of the Lower Cambrian or Olenellus Zone. Tenth Ann. Rept. Director U. S. Geol. Surv., 1888-89. 37. Woodward, Henry, 1895. Some Points in the Life-history of the Crustacea in Early Palaeozoic Times. Anniversary Address of the President. Quar. Jour. Geol. Soc. London, vol. li. 4. ON THE MODE OF OCCURRENCE AND THE STRUCTURE AND DEVELOPMENT OF TRIARTHRUS BECKI* (PLATE VI) THE presence of antennae and other appendages on a trilo- bite from the Utica Slate was announced in May, 1893, by W. D. Matthew, f The specimens were discovered by W. S. Valiant, J near Rome, New York, where they occur in a fine-grained carbonaceous shale. It was apparent that specimens preserving organs so delicate as antennae ought to show, in addition, other anatomical features which would be of great assistance in determining the zoological position of the Trilobita. With this object in view, and with the assist- ance of Professor Marsh, a collection was made for the Yale University Museum. From this material it is hoped that the remaining details in the structure of this important fossil may be made out. The preliminary examination of the speci- mens shows a number of new and remarkable structural features, some of which will be briefly noticed here. It was also possible for the writer to make observations in the field, which furnish interesting facts as to the mode of occurrence and to the habits of the trilobite. * Abstract of a paper "On the Structure and Development of Trilobites," read before the National Academy of Sciences, November 8, 1893. American Geologist, XIII, 38-43, pi. iii, 1894. t On Antennae and other Appendages of Triarthrus Beckii. Read before the N. Y. Academy of Sciences, May, 1893. Published in Amer. Jour. Sci. (3), XLVI, 121-125, August, 1893. J Mr. Valiant informs me that he found the first specimen showing antennae in 1884, but it was not until 1892 that other specimens were obtained by him and M. Sid. Mitchell fully establishing the discovery. The specimens sent to Columbia College were collected by W. S. Valiant, of Rutgers College. 198 STUDIES IN EVOLUTION In their present condition the specimens contain very little calcite matter, and nearly the entire calcareous and chitinous portions of the animal are represented by a thin film of iron pyrite. To this kind of fossilization is doubtless due the preservation of delicate organs and structures which other- wise would have been destroyed. For, as is well known, pyrite may replace such organic tissues as chitine or even soft dermal structures, the change occurring by the slow decomposition of these tissues in the presence of iron sul- phate in solution, or from the action of hydrogen sulphide as a result of decomposition in a chalybeate water. From the mode of occurrence of the specimens it is evident that some physical change of a rather sudden nature must be inferred to explain the facts. This is shown from the follow- ing considerations: (1) Their restricted vertical distribution; (2) nearly all specimens are complete and preserve their appendages; (3) they are of all ages, from larval forms up to full-grown individuals; (4) the rock has a characteristic structure and composition; and (5) the adjacent strata con- tain a rather sparse fauna in which the trilobites are generally fragmentary, or usually without appendages. It does not require a violent catastrophe to account for these peculiarities, and, as in the case of the recent destruc- tion of the tile-fish off the eastern coast of the United States, it is possible that a temporary change in the direction of an ocean current, with the consequent variation of temperature, would be amply sufficient. Just what occurred in the present instance has not been determined. Throughout the trilobite- bearing rocks generally, young and larval forms are extremely rare, while, of full-grown examples, fragments are the rule and entire specimens the exception. Therefore it is believed that the remains commonly found represent sheddings or moults, and not in each case the death of a separate indi- vidual. In the present material, however, the almost in- variable perfection of the specimens precludes this view. Moreover, the appendages are apparently in the position held in life, and not such as obtain in the cast-off shells of recent Crustacea. TRIARTHRUS BECKI 199 Another feature noticed in the field is that the specimens nearly all lie with the back down. The same thing has been observed by other investigators, and has been accounted for by the assumption that in being drifted about along the bottom such a position would be assumed from the centre of gravity being on the convex side. This idea does not seem tenable, because, while on their backs, the trilobites would be most easily rocked by the currents of water, and eventu- ally be turned over or dismembered. A further explanation has been offered by Hicks and accepted by Walcott,* to the effect that trilobites probably lived with the ventral side down, and the accumulation of gases in the viscera during decomposition was sufficient to overturn the animal and allow it to be buried by the deposition of sediments in the position now found. This theory, also, does not meet the facts as here observed, for in turning over a dead and limp animal provided with long and slender antennae, delicate jointed legs, and fringed appendages, the legs would be either folded under the carapace on one si4e, or displaced from their natural position. But, as has been already noticed, the present material generally shows the legs extended on both sides of the body and the antennae in a very lifelike position. (Plate VI, figures 3-7.) It seems most probable that trilobites could both swim freely and crawl along the bottom, and that, on dying, they coiled themselves up in the same manner as the recent isopods. Then upon unrolling they would necessarily lie on their backs. Even if they did not coil up, any swimming animal having a boat -shaped form would settle downward through the water with the concave side up. The definite structure of the legs of Triarthrus is now for the first time clearly shown, and is of much interest. Furthermore, a difference can be seen in the appendages of the pygidium, thorax, and cephalon. Those of the caudal * The Trilobite : new and old evidence relating to its organization. Bull Mus. Comp. ZooL, VIII, No. 10, 1881. 200 STUDIES IN EVOLUTION region overlap each other, and are furnished with very long hairs, or setae. The appendages of the head include the antennae and the mouth parts, the latter consisting of the mandibles and maxillae bearing palps and setae. The legs of the thorax have been worked out in detail, and are shown on Plate VI, figures 8, 9. No essential differ- ences have been observed in the series attached to the free segments. Each segment bears a pair of biramous append- ages, originating at the sides of the axis, as in other trilo- bites (Walcott, I. c.). The anterior legs are the longest, and the others gradually become shorter towards the pygidium. Those which are here taken for description are the legs of the second and third free thoracic segments. The entire length of the legs has been exposed from the dorsal side, by removing the overlying pleurae of the thorax, which con- cealed nearly half their length. Each limb consists of two nearly equal members, one of which was evidently used for crawling and the other for swimming. These two members and their joints may be correlated with certain typical forms of crustacean legs among the Schizopoda, Cumacea, and Decapoda, and may be described in the same terms. There- fore each limb is composed of a stem, or shaft, with an outer branch (exopodite) and an inner branch (endopodite). Plate VI, figure 9, shows the joints of the stem (6, 7), the exopo- dite (ex, 1 and 0), and the endopodite (en, 1-5). The pre- cise form of the coxal joint of the stem (coxopodite) has not yet been clearly made out. It is followed by a broad joint about twice as long as wide, which may be referred to the protopodite. The endopodite (figure 9, en) was the member used for crawling, as in the Schizopoda. The three proximal joints (5, 4-> &) are similar in form to 6, and taper gradually out- ward. The distal portion is completed by two slender cylin- drical joints (2, _Z), the latter bearing at its extremity short setae, or bristles, of which three are commonly to be seen. The other member, the exopodite (ex), lies over the en- dopodite. It apparently articulates with the protopodite, but TRIARTHRUS BE OKI 201 may spring from what is here referred to the coxopodite, as its basal portion is very broad and originates close to the articulation of the protopodite with the coxal joint. The proximal joint of the exopodite (#) is somewhat arched and tapers rapidly. It extends to the ends of the pleurae, and is the longest joint of either branch. The posterior edge is finely denticulate, and carries a row of long setae. The distal portion (.Z) is multiarticulate, being composed of ten or more joints. In general form it is slightly crescentic, with the margins thickened, the anterior one being strongly crenulated. Long setae extend posteriorly from the crenu- lations on the dorsal side of the leg, making a conspicuous fringe along the distal half of the exopodite. Plate VI, figure 7, represents a dorsal view of Triarthrus Becki, showing the antennae and the exposed portions of the appendages. The antennae and legs on the right side are drawn from one specimen, and the legs on the left side are as shown in another individual. The biramous character of the entire series of thoracic legs is f very evident, as is also the distinction between the crawling and swimming members. Figure 8 shows the right second and third legs of the free thoracic segments. In figure 9 the upper exopodite is repre- sented without setae, so as to bring out the structure in greater detail. On the lower leg the setae are shown. The antennae are about as long as the head, and are com- posed of short conical joints. They usually occur in the position shown in figures 5 and 7, but occasionally lie close to the margin, figures 3 and 4, and sometimes curve back- ward over the head, as in figure 6. It is not necessary in this place to describe in detail the development of Triarthrus Becki, but attention may be called to two early larval forms. The youngest is shown on Plate VI, figure 1, and may be compared with the first segmented stage, figure 2, and with the adult, figure 7. At this early stage the animal is less than one millimetre in length (.63 mm.), and has no distinct separation into parts. The divi- sion into a cephalic and a caudal region is indicated by a 202 STUDIES IN EVOLUTION transverse groove, but as yet the body segments are undevel- oped. After the separation of the head and pygidium the thoracic segments are introduced successively between the head and abdomen until the full number is reached, and the animal measures from 10 to 55 millimetres in length. The segmented stages have been described fully by Walcott, * and an outline figure of the stage with one thoracic segment is given in figure 2. The final conclusions to be reached from a complete study of the development and structure of these animals can as yet ba only surmised. It is quite evident, however, that they are related to the true Crustacea. The Trilobita are shown to be a primitive type in (1) their multiple segmentation, (2) the irregular number of thoracic legs, and (3) the biramous structure of the legs. They therefore present characters common to the Entomostraca and Malacostraca. * Trans. Albany InsL, X. 5. FURTHER OBSERVATIONS ON THE VEN- TRAL STRUCTURE OF TRIARTHRUS* (PLATES VII and VIII) IN previous papers on the ventral structure of Triarthrus^ the anterior antennae, thoracic legs, and appendages of the pygidium have been described.! There yet remain for inves- tigation the appendages of the head and additional details of other parts of the animal. These characters have not been easily obtained on account of the labor of removing the rock from such delicate structures. Moreover, but few specimens are in the proper position or condition to yield satisfactory results. The appendages of the bead either suffered greater decomposition than those of the thorax, before mineraliza- tion, or were so tenuous as to be easily obliterated, and are now seldom sufficiently well preserved for study. Further, the number and compact arrangement of such complicated organs, even when present, make it difficult to trace their precise form. A similar difficulty would be experienced were one to endeavor to describe the appendages of Apus by examining the ventral side without cutting away some of the limbs or at least unfolding or bending them around. * American Geologist, XV, 91-100, pis. iv and v, 1895. t W. D. Matthew. On Antennae and other appendages of Triarthrus Beckii. N. Y. Academy of Sciences, May, 1893 ; Amer. Jour. Sci., August, 1893. C. D. Walcott. Note on some Appendages of the Trilobites. Proc. Biol. Soc. Washington, March, 1894. C. E. Beecher. On the Thoracic Legs of Triarthrus. Amer. Jour. Sci., December, 1893. On the mode of Occurrence, and the Structure and Development of Triarthrus Becki. American Geologist, January, 1894. The Appendages of the Pygidium of Triarthrus. Amer. Jour. Sci., April, 1894. 204 STUDIES IN EVOLUTION The features described in the present paper have been ob- tained by further work on the material secured for the Yale Museum by Professor Marsh. No detailed review of the ventral anatomy of Triarthrus will be given at this time, only such additional characters as have been observed since the publication of the last paper on this trilobite. The precise structure and relations of the organs here described must also be left subject to slight modifications required by researches which are still in progress. The writer has care- fully prepared the specimens and made the drawings from camera-lucida outlines. The appendages, however, are often so faintly preserved or so obscure that in order to represent them in a pen-drawing it is necessary to emphasize their limits and their prominence, and this may sometimes lead to errors of interpretation. It seems almost unnecessary to state that errors are not due to any preconceived notions of trilobite anatomy, since from the beginning of these investi- gations it has not been possible to predict with safety the exact form and details of any of the appendages. Even their presence has been more or less doubtful until revealed by a fortunate discovery. The paired appendages of the cephalon will be taken up in their order, beginning with the most anterior ; next the newly observed characters of thoracic legs ; then the organs in the median line, the hypostoma, mouth, metastoma, and anal opening. Close observation of the specimens thus far prepared for the purpose of showing the under side of the head fails to detect more than five pairs of appendages attached to the cephalon, apparently corresponding to the five typical limbs of the crustacean head. Considerable difficulty is experi- enced in determining from the ventral side of the specimens the posterior limit of the cephalon. The ventral membrane, which alone is usually visible, does not show marked evi- dence of segmentation, and the observer is guided chiefly by the margin of the cephalon, the extremities of the pleura, and obscure transverse lines on the axial membrane. In a VENTRAL STRUCTURE OF TRIARTHRUS 205 few cases the evident sliding or displacement of the dorsal and ventral surfaces further complicates the attempt to refer the appendages to definite divisions of the animal. Paired Uniramose Appendages. Anterior Antennae, or Antennules. These have been de- scribed by Matthew, Walcott, and the writer (I. .). Wal- cott showed their proximal extremities and their mode of attachment at the side of the hypostoma. Little more can now be added except that they are evidently the first pair of antennal organs, and correspond to the antennules of other Crustacea. The strong basal joint or shaft is shown in Plate VIII, figures 9, 10, 11, attached to the ventral side of the head at each side of the hypostoma, near the middle of its length. The shaft carries a single flagellum, and thus agrees with the typical uniramose antennule of the nauplius of Crus- tacea. This simple antennule is still present in the Isopoda, as in Mannuopsis typica. The flagella curve forward and ap- proach, nearly touching as they cross the doublure. Beyond the limits of the head they are variously disposed, though usually extending forward, at first diverging for half their length and then slightly converging (Plate VIII, figures 5, 6, 7). Paired Biramous Appendages. The remaining paired appendages of the trilobite all seem to be biramous, and agree closely in their general features. Adjacent members of the series present very slight differ- ences. It is only when the primitive and simple phyllopodous legs of the pygidium are compared with the anterior thoracic or cephalic appendages that variations of note can be ob- served, although these are of form and not of structure. On this account there is no well-defined separation into pos- terior antennae, mandibles, maxillae, maxillipeds, thoracic, and pleopodal appendages. It is most convenient, therefore, to number them from before backward, and to indicate 206 STUDIES IN EVOLUTION homologous positions with other Crustacea only when there is some evident reason for so doing. First Pair of Biramous Appendages, or Posterior Antennae. The second pair of appendages, corresponding to the posterior antennae, are attached to the head at each side of the glabella, on a line with the extremity of the hypostoma. They are apparently biramous, and thus agree with the second pair of nauplian limbs and with the typical posterior antennae of many Entomostraca and Malacostraca. They may be compared with the posterior antennae in EupJiausia pellucida, one of the schiz- opods, especially with the Furcilia and Cyrtopia stages. The details of the endopodite and exopodite are not clearly shown. The former is more commonly preserved, and its distal joint extends just beyond the edge of the carapace. The coxopo- dite is developed into a triangular plate, the inner angle carrying a masticatory ridge, the whole extending about three-fourths the distance from the side of the glabella to the median line, just below the hypostoma, and directed obliquely backward (Plate VIII, figures 8-11). Second Pair of Biramous Appendages, or Mandibles. The appendages here correlated with the mandibles are immedi- ately behind the first pair of biramous limbs. The proximal por- tion, or coxopodite, is similar in form to the preceding, though somewhat smaller, and overlapping its basal part. The palps, or endopodial and exopodial branches, have not been distinctly traced, though their presence is indicated on Plate VII, fig- ure 1, where, on the left side, there are endopodites and exop- odites in sufficient number for each appendage of the head. That these should be referred to the cephalic limbs is further indicated by their being in advance of the endopodite, which manifestly pertains to the first thoracic segment. The inner edge of the mandibles as well as that of the other gnathobases of the head is apparently finely denticulate, as shown on Plate VII, figure 1, and Plate VIII, figure 2. Third and Fourth Biramous Appendages, or Maxillae. Following the appendages referred to the mandibles are two pairs of strong limbs, with broad plate-like basal portions, or VENTRAL STRUCTURE OF TRIARTHRUS 207 coxopodites, serving as gnathites (Plate VIII, figures 8-11). They resemble each other, and are similar in form to the two preceding limbs, though somewhat larger. They are usually fairly well preserved, and their form and structure can be approximately made out. The endopodites are composed of stout joints, and could be extended but a short distance beyond the margin of the head. The exopodites are more slender, and carry an abundance of stiff setae, which often diverge in a fan-like manner from their line of attachment. These brushes of setse occupying the cavities of the cheeks are often preserved in specimens where the other details of the limbs are obscure or obliterated. In Triarthrus they are evidently homologous with similar brushes observed by Wal- cott in Calymmene.* This completes the number of paired appendages which can be definitely referred to the head. It is evident they do not differ conspicuously from each other, and, as will be presently shown, they closely resemble the thoracic legs in all essential structural characters. , Thoracic Legs. In the paper by the writer (I. eo9 young, and rprjfjia perforation.) (Plate XI, figures 5-12.) Protegulum as in the preceding order in primitive forms, becoming more circular and with shorter and more arcuate hinge in the pedicle valve of derived types. Growth of the dorsal valve tending to become peripheral. In the opposite valve the pedicle more or less surrounded by progressive neanic growth posterior to the initial hinge. Pedicle fissure remaining open in primitive mature forms, becoming enclosed in secondary forms during neanic stages, and in derived types enclosed in early neanic or nepionic stages. Valves inarticulate. 1 Including the genera : Anvistocrania. *Disdnopsis. Pholidops. Acrothele. Helmersenia. Pseudocrania. Acrotreta. Kayserlingia. *Roemerella. *Conotreta. Lindstroemella. *Schizambon. * Crania. *Linnarssonia. * Schizobolus. *Oraniella. Mesotreta. *Scliizocrania. Craniscus. *(Ehlertella. Siphonotreta. *Discina. * Orbiculoidea. *Trematis. *Distinisca. PROTREMATA. (Trpco early, and rprj^a perforation.) (Plate XI, figures 13-21.) Protegulum of the dorsal valve as in the Atremata. In the ventral valve it has become modified to an elliptical or circular form with arcuate hinge. Pedicle enclosed in early nepionic stages by a pro-deltidium ; posterior covering (deltid- ium) retained at maturity, or resorbed or abraded in neanic stages, so that the pedicle protrudes between the two valves. Valves articulate. DEVELOPMENT OF THE BRACHIOPODA 245 Including the genera : Ampliigenia. Aulosteges. Bactrynium. BiloUtes. Camarella (group). Camarophoria. *Chonetes. Clitambonites. Conchidium. Davidsonella. Davidsonia. Daviesiella. Derbya. Enteletes. Eudesella. Hemipronties. Hipparionyx* *Lacazella. *Leptcena. Leptcenisca. Lyttonia. Meekella. Mimulus. Oldhamina. *0rthis (group). Orthisina. *0rthothetes. Pentamerella. Platystrophia. *Plectambonites. Porambonites? Proboscidetta. *Productella. Productus. * Rhipidomella. Schizopkoria. Sieberella. Streptis. *8treptorhynchus. Stricklandinia. Strophalosia. * Stroplwodonta. * Strophomena. * Strophonella. ThecideUa. * Thecidium. Thecidopsis. Triplecia. TELOTREMATA. (reXo? last, and rp^^a perforation.) (Plate XI, figures 22-28.) Protegulum as in Atremata. Pedicle-opening shared by both valves in nepionic stages, usually confined to one valve in later stages, and becoming more or less limited by two deltidial plates in ephebic stages. Arms supported by cal- careous crura, spirals, or loops. Valves articulate. Including the genera : Acanthotliyris . Ambocoe.Ua. Amphidina. *Athyris. *Atretia (Cryptopora). *Atrypa. Bifida. Bouchardia. Centronella. *Cistella. Clorinda. * Ccelospira. Ccenothyris. Cryptonella. Cyrtia. Cyrtina. Dayia. Dictyothyris. Dielasma. Dimerella. Disculina. Eatonia. Eudesia. Eumetria. Glassia. Grunewaldtia. *Hemithyris. Hindella. Ismenia. Karpinskya. Kayseria. Kingena. Koninckella. * Koninckina. *Jraussina. *Laqueus. 246 STUDIES IN EVOLUTION Leptocodia. Nucleospira. Stringoceplialus. Leiorhynclms. Pentagonia. Suessia. *Liothyrina. Peregrinella. Syringothyris. *Macandrevia. Platydia. * Terebratella. Magas. Rensselceria. Terebratula. *Magellania. Reticularia. * Terebratulina. *Martinia. Eetzia. Terebratuloidea. Martinopsis. * Rliynclionella. Tliecospira. Megatliyris. Rliynclionellina Trematospira. Megalanteris. Rhynclioporina. Trigonosemus. *Megerlina. Wiynchotrema. *Tropidoleptus. Merista. * Ehynchotreta. Uncinulus. *Meristella. *JSpirifer. Uncites. *Meristina. Spiriferina. Zellania. *Muhlfeldtia. Spirigerella. *Zygospira. PART II. CLASSIFICATION OF THE STAGES OF GROWTH AND DECLINE* (PLATE XII) A BRIEF review of the known embryology of the Brachiop- oda is desirable, in order to account for some of the differ- ences presented by adult forms in the several divisions of the class. This knowledge is far from complete, and is confined to a few species, but much of interest bearing on the later development of the organism may be obtained. The important memoirs ( of Morse, 18 ' 19 Kovalevsld, 15 Lacaze- Duthiers, 16 and Shipley 22 contain nearly all that is known regarding the early embryology of brachiopods. The genera included in the works of these authors comprise Cistella, Terebratulina, Liothyrina, and Lacazella. Later larval stages of the genus G-lottidia have been fully described by Brooks. 4 Miiller, 20 also, has given a description and figures of a larval form doubtfully referred to Discinisca. The results of these observers must at present be taken without reservation, and are thus made use of in the present paper. * Amer. Jour. Set. (3), XLIV, 133-155, pi. i, 1892. t The works referred to by numbers are cited in full in the list appended to this article. DEVELOPMENT OF THE BRACHIOPODA 247 Something is known, therefore, of the early stages in each of the four groups or orders proposed by the writer. 2 The Atremata, Neotremata, arid Protremata are represented by a single genus only in each; Grlottidia, Discinisca, and Laca- zella, respectively; and the Telotremata, by Cistella, Tere- bratulina, and Liottiyrina. Were Grlottidia and Discinisca as well known as Cistella, Terebratulina, and Lacazella, some comparisons could undoubtedly be made which would en- lighten many obscure points of anatomy and morphology, as well as give clearer insight into the history and origin of each group. Cistella and Terebratulina are taken as standards of the embryological development on account of the completeness with which they have been studied, and because their points of difference are not great. Lacazella shows such peculiar features that its history must be discussed separately. The nepionic Grlottidia and Discinisca, too, present characters which evidently had an early history somewhat different from Cistella or Terebratulina. * In taking up the review of the observed stages of growth, an attempt will be made to fix their limitations. To this end the admirable nomenclature proposed by Hyatt 9> 10 is here adopted, as it is more convenient and of wider applica- tion and significance than the terms heretofore used. Thus far this system has been employed principally in studies relat- ing to the Mollusca, and its application to the Brachiopoda will necessarily require some illustration and explanation. In the preface to " Genesis of the Arietidse " Hyatt has pre- sented a summary of the theoretical opinions resulting mainly from his studies in the Cephalopoda. It is believed that nearly the same ground may be covered in the Brachiopoda, and thus the truth of these deductions will receive further evidence from another class of organisms. Embryonic Stages. The true embryonic stages are classified by Hyatt as Pro- tembryo, Mesembryo, Metembryo, Neoembryo, and Typembryo* 248 STUDIES IN EVOLUTION To these Jackson n has added the PTiylembryo, taking it from the later stages of the Typembryo to represent the period when the animal can be referred definitely to the class to which it belongs. The succeeding stages in the growth of the animal to maturity are termed by Hyatt [and emended by Buckman and Bather] nepionic (young), neanic (adolescent), and ephebic (mature), while old-age characters are called gerontic. The stages are further divided by using the prefixes ana, meta, and para; as anagerontic, metagerontic, and paragerontic. The application of this nomenclature of the stages of growth and decline to the Brachiopoda is shown on the fol- lowing pages. 85 86 87 Cistella neapolitana Scacchi. FIGURE 85. Protembryo; unsegmented ovum. FIGURE 86. Protembryo ; ovum composed of two spheres. FIGURE 87. Mesembryo ; blastosphere. FIGURE 88. Metembryo; gastrula. (Figures 85-88, after Shipley.) The Protembryo, as in other groups of organisms, includes the ovum and its segmented stages preceding the formation of a blastula cavity. Figures 85 and 86 show protembryonic stages of Cistella. The eggs are spherical, pyriform, or ovoid, and the segmentation proceeds in a regular manner, resulting in a blastosphere composed of equal parts. The Mesembryo, or blastosphere (figure 87), has been ob- served in Cistella, Terebratulina, and Lacazella. The blas- tula cavity is small. The Metembryo, or gastrula stage (figure 88), is developed from the blastosphere in two ways : (a) by embolic in vagina - tion in Cistella and Terebratulina (Kovalevski and Shipley), and (5) by delamination in Lacazella (Kovalevski). At the close of this stage the archenteron in Cistella is trilobed, DEVELOPMENT OF THE BRACHIOPODA 249 consisting of a central cavity, or mesenteron, connecting on each side with the body cavity. The Neoembryo, represented by the trochosphere and seg- mented, ciliated, cephalula stages, has been more fully ob- served than any of the preceding. The first advance from the completed gastrula is in the separation of the mesenteron from the body cavity, and the division of the organism into two segments or lobes, the cephalic and caudal (figure 89). Later a third or thoracic segment is developed and carries four bundles of stiff barbed setse (figure 90). The cephalic and caudal lobes are densely ciliated. During the subse- quent cephalula period two eyes, then two others, appear in Cistella, and at the same time the dorsal and ventral sides of the thoracic segment become extended over the caudal, and are progressively defined as two lobes (figures 89-93, 108, 109). 89 90 91 92 Cistella neapolitana Scacchi. FIGURE 89. Neoembryo ; embryo of two segments. FIGURE 90. Neoembryo ; cephalula ; ventral side ; showing cephalic, thoracic, and caudal segments, eye-spots, and bundles of seta3. (Figures 89 and 90, after Kovalevski.) FIGURE 91. Neoembryo; lateral view of completed cephalula stage; show- ing extent of dorsal (d) and ventral (v) mantle lobes, and umbrella-like cephalic segment. FIGURE 92. Neoembryo; same stage; ventral view. (Figures 91 and 92, after Shipley.) Terebratulina has a tuft of bristles on the top of the cephalic segment. In Lacazella the bundles of set83 are absent, and the head is more distinctly differentiated from the anterior segment than in Cistella. The closing cepha- lula stage in Cistella has an umbrella-like expansion of the 250 STUDIES IN EVOLUTION cephalic border, and the organism becomes a free-swimming larva (figures 91-93). Larval Stages. The Typembryo is the larval stage at which some distinc- tive features make their appearance, but before the special characters of the class are to be found (figure 94). It is analogous to the molluscan embryo in which a shell gland and plate-like initial shell are developed. There is, however, no homology of parts or organs between the t}~pembryonic mollusk and brachiopod. Cistella neapolitana Scacchi. FIGURE 93. Neoembryo ; completed cephalula stage. FIGURE 94. Typembryo; transformed larva resulting from folding up- ward of mantle lobes over cephalic segment, ad, muscles from bundles of setse to sides of body cavity ; di, muscles from dorsal to ventral sides of body ; vp, muscles from ventral side of body to caudal segment or pedicle. (Figures 93 and 94, after Kovalevski.) In Cistella and Terebratulina the development of the typ- embryo has been observed, and consists of the folding upward of the lobes which have been developed from the thoracic segment to form the mantle, so that they gradually DEVELOPMENT OF THE BRACHIOPODA 251 enclose the anterior end (figures 108-111). The surfaces of the mantle which were exterior in the cephalula have now become inner and the bundles of setse have revolved 180, changing their direction from posterior to anterior. This leaves the lower part of the thoracic, and the whole of the caudal, segment exposed. The outer surface of the mantle is invested with a hard integument, which, upon completion and before the growth of the true shell, forms the protegu- lum. The pedicle at this stage is also defined, being a modi- fication of the caudal segment. It may serve to attach the larva to foreign objects, as in Cistella (figure 94) and Tere- bratulina, or it may remain undeveloped for a time, as in Grlottidia and Discinisca. A rudimentary digestive tract is present. The body muscles which have been developed thus far consist of four distinct pairs. Two pairs lie close to the sides of the body cavity, and extend to the points of inser- tion of the bundles of bristles (figure 94, ad). They become after transformation the four adductor muscles of the valves. The third pair extends from the ventral side of the body to the caudal segment, and is converted into the ventral pedicle muscles (figures 94, 99, 100, vp). The fourth pair is situated posterior to the digestive tract, and extends from the dorsal to the ventral wall of the body (figure 94, di). They form the divaricator muscles in the mature brachiopod (figure 100, di), and are divided into or duplicated by a pair of dorsal and a pair of ventral divaricators. There is also a pair of dorsal pedicle muscles in the larva of LiotJiyrina and Terebratulina. The folding upward of the mantle lobes forms the first hinge-line of the future valves (hi, figures 110, 111). Thus its origin is not, as in pelecypods, a line produced by the bending of a single plate (Jackson), but is the line along which the two mantle lobes are bent against the body. Between them projects posteriorly nearly half the body of the animal, and the whole opening corresponds to the pedicle- opening of later stages of growth. The hinge of brachiopods, 252 STUDIES IN EVOLUTION therefore, is not primarily a line of articulation of the valves, but the limiting borders between the body and the attached edges of the mantle. Secondarily, and during later growth, the extension of the valves along a line of apposition forms a true hinge-line. The first points of contact of the valves to form the true hinge lie adjacent to the right and left sides of the body of the animal, at the cardinal extremities (figure 99, t). Here naturally the first hinge-teeth are formed, and their position corresponds to that in adult individuals; namely, on each side of the cardinal opening. The enlarging of the cardinal opening by shell growth results in the gradual divergence or separation of the teeth, as in Terebratulina. In species with extended hinge-lines, as in many forms of Spirifer, Orthis, and Strophomena, the teeth still lie in their original position on each side of the cardinal opening, and the elon- gation of the hinge has come not only from the enlargement of the opening by growth, but by additions at the hinge extremities, so that the teeth are situated on each side of the central area, below the beak, and not at the cardinal angles. The young of these genera, however, all have the hinge-teeth at the extremities of the hinge, as the cardinal opening then occupies the whole posterior area of the shell. Adult specimens of Kutorgina (K. cingulata Billings) have a deltidium as in Strophomena. The cardinal opening in- cluding the deltidium occupies the whole posterior end of the shell, and according to a statement made to the writer by Mr. Charles Schuchert, there are rudimentary teeth at the cardinal extremities. Therefore this genus represents a nepionic condition of later forms, and, on account of these and other characters, it is believed to be related to Orthisina and Strophomena, of which it is the ancestral type. It con- sequently belongs to the articulate brachiopods. The embryonic stages up to this point have frequently been compared to similar stages in other organisms, especially in the Annelida and Polyzoa. Without repeating these com- DEVELOPMENT OF THE BRACHIOPODA 253 parisons, which may be consulted elsewhere, 4 ' 12 15> 16 ~ 19 ' 21 attention is called to the similarity of development of the brachiopod typembryo to the larval stages of Spirorbis. There are, however, important structural differences. An article by J. W. Fewkes, " On the Larval Forms of /Spirorbis borealis Daudin," 7 contains a nearly complete and very inter- esting account of the development of this chsetopod. There is a striking resemblance in the characters of the cephalula 98 96 Spirorbis borealis Daudin. FIGURE 95. Cephalula, developing lobe from the body (col). FIGURE 96. More advanced stage. FIGURE 97. Larval form before transformation ; showing posteriorly directed expansion (col) from thoracic segment. FIGURE 98. Transformed Spirorbis ; showing folding upward of collar partially enclosing head. (Figures 95-98, after Fewkes.) stages in both organisms, as may be seen on comparison (figures 95 and 96). Spirorbis develops a posteriorly directed extension from the middle segment, called a collar, which in later stages is reflexed anteriorly so as to cover more or less the cephalic portion, thus agreeing with the growth and change in position of the mantle in Cistella. The ventral lobe is also the larger in both. Many other comparisons and homologies have been made by Morse, 19 and the one here described is even more marked than his reference to the 254 STUDIES IN EVOLUTION lobation of the cephalic collar in Sabella. Four figures are introduced illustrating the principal changes in Spirorlis. They may be compared with the development of Cistella shown in figures 9094. It is not intended by this to indicate a close relationship with the chsetopods, for the writer is inclined to accept the opinion of Joubin, 12 that the brachiopods constitute a distinct and independent class. The Phylembryo (figure 99) differs from the typembryo in (a) the completion of the embryonic shell, or protegulum; (6) the first appearance of the tentacular lobes of the lopho- v f Cistella neapolitana Scacchi. FIGURE 99. Phylembryo; brachiopod; showing shell (protegulum), begin- ning of tentacles of lophophore (/), obsolescence of eye-spots, and formation of oesophagus ; t, hinge-teeth ; vp, ventral pedicle muscles. FIGURE 100. Nepionic brachiopod ; showing distinct tentacles of lophophore, mouth, and stomach, and transformation of muscles from typembryo (figure 94). ad, adductors; di, divaricators ; vp, ventral pedicle muscles. (Figures 99 and 100, after Kovalevski.) phore, or arms; (V) the usual dehiscence of the four bundles of setse ; (d) the obsolescence of the eyes ; (e) the definition of the oesophagus and stomach, and (/) the agreement of the muscular system with that in adult forms. These fea- tures, with the pedicle which appeared in a preceding stage, represent the brachiopod phylum, and are properly referred to the phylembryonic period of Jackson. Although the molluscan stage called the prodissoconch in pelecypods, the DEVELOPMENT OF THE BRACHIOPODA 255 protoconch in cephalopods and gastropods, and the periconch in scaphopods, represents the completed phylembryo of these groups, as the protegulum represents a like period in the developing brachiopod, yet there is no homology of distinc- tive organs. The mantle of mollusks is first formed on the posterior dorsal side, and is in the shape of a disk, which gradually envelops the animal to a greater or less extent, and may become distinctly lobed. As has been shown, this organ in the brachiopods develops simultaneously from the dorsal and ventral sides of the thoracic segment of the cephalula, and is primarily bilobed. The initial shell of brachiopods is not produced from a dis- tinct shell gland, as in the Mollusca, but is an integument of the surface of the mantle lobes, and intimately connected with them. The position of the valves is dorsal and ventral. The pedicle has no organic similarity with either a foot or a byssus. The mouth of mollusks (and annelids) is formed below the base of the cephalic lobe of the cephalula, and may be the blastopore, while in the brachiopods it is near the anterior pole within the cephalic segment. Notwithstanding these differences, so many parts are functional equivalents that their growth and development may be discussed and inter- preted in the same terms. Before passing to later stages of growth which become more and more divergent from a common simple type, some points previously omitted, relating to Thecidium (Lacazella), Lingula (Grlottidia), and Ditcinitca, should be here noted. As Lacazella is a form in which the ventral valve in the neanic and ephebic stages is cemented to foreign objects by calcareous fixation, it bears about the same relation to other brachiopods that Ostrea bears to Avicula, among the pelecy- pods, and a corresponding early absence or modification of many features present in adult individuals should be looked for. From what is known of the geological history of The- cidium, and if the interpretations of its phylogeny by the 256 STUDIES IN EVOLUTION writer are correct, it is derived from an ancestry which had a similar condition of fixation as early as the Upper Silurian. Thecidium is apparently not a terebratuloid genus. Its struc- tural affinities are evidently with the strophomenoids, espe- cially such forms as Plectambonites, Leptoenisca, etc. Briefly the reasons for this statement are (a) the presence of a del- tidium of one plate ; (6) the absence of a true loop supporting the arms (the internal calcification or spiculization is confined wholly to the mantle, and does not extend to the arms 16 ) ; (c) a concave plate in the cavity of the ventral beak, bearing the divaricator muscles ; (d) the attached ventral valve, and (e) the cardinal processes in the dorsal valve.* The first character is of prime importance, because all the strophom- enoids and none of the terebratuloids have a deltidium of one plate. It would appear, therefore, that the early, free-swimming, larval state, and the later pediculate stage have become lost by acceleration, thus accounting for the very unequal develop- ment of the mantle lobes in the cephalula stage, and the non- active and early sedentary larvae as described by Kovalevski and Lacaze-Duthiers. The young Lingula (G-lottidia) described by Brooks, and the Discinisca by M tiller, 20 both representing the phylem- bryonic stage, were active and free-swimming animals, with rudimentary pedicles. Terebratulina becomes attached or rests on the caudal segment during the cephalula stage (Morse), while at the end of this period in Cistella (Kova- levski and Shipley) there is an active, swimming, ciliated organism, which later attaches itself by the pedicle in the typembryonic period. From the facts that young individuals of Paleozoic species belonging to such genera as Zygospira, Spirifer, Orthis, Rhynchonella, and Scenidium, have been observed by the * Dall in 1870 (Amer. Jour. Conchology) made a clear statement of the characters of Thecidium and of many of its radical points of difference with the Terebratulidae, showing that it was entitled to rank as the type of a distinct family. DEVELOPMENT OF THE BRACHIOPODA 257 writer to retain their original relations to the objects of sup- port, and that casts of the pedicles of fossil Lingulse and Eichwaldia have been described (Davidson, 5 Walcott 23 ), it cannot be assumed that the free-swimming condition was ever present in neanic or ephebic individuals. Evidently it has always been a larval character. Origin of the Deltidium and Deltidial Plates. The origin and significance of the deltidium * (= " pseudo- del tidium ") are made apparent in the development of The- cidium, and it may be well in this place to make a few observations on the genesis of this important character, and its relations to the deltidial plates of other genera, as Rhynchonella and Terebratula. It has been already noted (Part 1), that the deltidium in all species possessing it (the Protremata) is an embryological, or nepionic feature, which may or may not continue to the ephebic period; while the deltidial plates in other brachiopods (the Telotremata) appear later during the neanic and ephebic periods, or may never be developed. The detailed researches of Kovalevski on Cistella and Thecidium, together with other observations now first made, furnish data for a clear understanding of these differences. J Figure 102 represents a dorso-ventral section of a ripe cephalula just before the transformation, and shows the un- * The single plate or covering to the triangular opening beneath the ventral beak should be termed the deltidium, as it was thus extensively used by David- son. When it consists of two plates, they may be called deltidial plates. These names have been loosely used. In Part I of this paper the deltidium proper is referred to as pedicle covering, pedicle-sheath, and pseudo-deltidium. Hall and Clarke have proposed to call the triangular opening in the beaks of brachiopods the delthyrium, and the concave plate in the ventral beak of Pentamerus, Orthisina, etc., they have termed the spondylium. There yet remains a term for the convex plate covering the opening below the beak of the dorsal valve, and resembling the deltidium of the opposite valve. For this feature the name chilidium (x^Aos) is here proposed. J Kovalevski 15 For Thecidium consult the explanation of pi. iv, figs. 15-26 ; for Cistella, pi. i, figs. 13-15; pi. ii, figs. 17, 19-21. 17 258 STUDIES IN EVOLUTION equal lobes of the mantle, v being the ventral lobe, and d the dorsal; h is the head, and^? the caudal segment develop- ing into a pedicle. A deposit of integument representing Thecidium (Lacazelld) mediterraneum Risso. FIGURE 101. Cephalula; dorsal side, ds, dorsal shell plate; h, head. (After Kovalevski.) FIGURE 102. Dorso-ventral longitudinal section of cephalula of about same age as preceding, h, head ; d, dorsal mantle lobe ; v, ventral mantle lobe ; ds, beginning of dorsal valve; del, shell plate forming on dorsal side of body; p, pedicle. (After Kovalevski.) FIGURE 103. Typembryo; larva transformed from folding upward of mantle lobes, h, head ; ds, dorsal valve ; hi, hinge-line of dorsal valve ; del, shell plate on body and pedicle posterior to hinge-line of dorsal valve. (After Kovalevski.) FIGURE 104. Dorso-ventral longitudinal section of preceding. References as in figure 103. vs, ventral valve; p, pedicle. FIGURE 105. Profile view of neanic Leptcena rhomboidalis. The features of the shell are placed and lettered as in figure 104. ds, dorsal valve; hi, hinge- line ; del, deltidium ; p, pedicle-opening ; vs, ventral valve. FIGURE 106. Adult Thecidium (LacazeUa) mediterraneum; dorsal side; showing ventral area and deltidium. FIGURE 107. Profile of same. References as in figures 104 and 105. the shell has formed on the inner side of the dorsal mantle lobe (c?s), and also on the adjacent dorsal side of the body lobe (deT). A larva somewhat more advanced is represented in figure 101, as viewed from the dorsal side. The mantle DEVELOPMENT OF THE BRACHIOPODA 259 lobe is still directed posteriorly, as in the preceding figure, and the underlying shell plate is shown at ds. In the process of transformation (figures 103, 104), the mantle lobe is turned forward in the usual manner, bringing the shell on the outside of the animal, so that both dorsal plates are now exposed, ds being the dorsal valve, and del the shell devel- oped on the dorsal side of the walls of the body and caudal segments. As this plate (del) is below or posterior to the hinge-line (^Z), and extends down over the pedicle, it is evidently the beginning of the deltidium. At the same time there is an extension of the edges of the mantle and pedicle on the ventral, or lower, side and shelly matter is deposited, forming the ventral valve (vs, figure 104). At this stage the hinge-line (figures 103, 104, hi) is the line between the dorsal mantle shell (ds) and the dorsal body shell plate (del). The beak of the ventral valve is separated from the dorsal beak by the pedicle and the shell covering to the pedicle and body lobe, or the deltidium. The valves afterward meet at their peripheries ; the hinge is extended beyond the deltidium, forming the true hinge of articulate brachiopods. As there is no motion between the ventral valve and the deltidium, the two become ankylosed. Figures 106 and 107, showing an adult Thecidium, are lettered in the same manner as the pre- ceding, and express the same relation of parts. The deltidium is not, therefore, primarily, on account of its manner of origin, an integral part of the ventral valve, but is a shell growth from the dorsal side of the body, which afterward becomes attached to the ventral valve, and is then considered as belonging to it. The further growth of the deltidium around the body and pedicle, and its consequent extension into the cavity of the ventral umbo, may explain the origin of the spondylium. Kovalevski 15 believed the ventral valve in Thecidium was secreted by the expanded edges of the pedicle and the body walls ; whether or not this is so does not affect the interpre- tation of the origin of the deltidium. From the observations of Lacaze-Duthiers, 16 it seems, however, as though the ven- 260 STUDIES IN EVOLUTION tral mantle lobe must have formed the shell in the usual way. This appears all the more probable from the fact that the lower or ventral valve is punctate, and, so far as known, the mantle contains all the csecal prolongations, which alone could produce the punctate structure. Careful microscopic examination has failed to detect punctse in the deltidia of Thecidium, Strophomena, Leptcena, and other punctate genera belonging to the Protremata. It is true that Aulosteges has spines on the deltidium, but spines even when tubular are not equivalent to punctse, as shown in Producing Strophalosia, and some species of Spirifer. Aulosteges is a gerontic genus, which has become excessively spinose, and has also reverted to ancestral char- acters in its high hinge-area and conspicuous deltidium. It is well known that even the spires of Spiriferina and the loop of Macandrevia are spinose. Turning now to Cistella as a representative of the Telo- tremata, a different process obtains. Figure 108 represents the fully developed, free-swimming cephalula of Cistella, and shows the extent of the folds of the mantle and their posterior direction. Figure 109 repre- sents the same in section. The inner sides of the mantle lobes are to form the future valves, the dorsal (ds\ and the ventral (vs) . The transformed larva or typembryo is repre- sented in figure 110 and in section in figure 111. It is seen that the transformation consists in the folding forward of the mantle lobes over the head segment (A). Now the shell- secreting layers of the mantle are exterior, and the two valves begin to form, the dorsal shell (c?s), and the ventral (vs). The pedicle and posterior portion of the body come out freely between the valves and mantle lobes and limit the hinge-areas of both (hi and M). The further process of growth increases the distance be- tween the initial dorsal and ventral hinges, for while the original dorsal beak is usually maintained at the hinge-line, the ventral beak is progressively removed and the ventral hinge travels from its first position at the beak, along the DEVELOPMENT OF THE BRACHIOPODA 261 edges of the umbo, leaving an open triangular area or del- thyrium in the ventral valve occupied by the pedicle. This 108 Cistella neapolitana Scacchi. FIGURE 108. Lateral view of completed cephalula stage, h, head ; d, dorsal lobe of mantle; v, ventral lobe; p, pedicle. (After Shipley.) FIGURE 109. Dorso-ventral longitudinal section of same : showing posteri- orly extended mantle lobes. A, head ; ds and vs, inner surfaces of mantle lobes which are to form dorsal and ventral valves. (After Shipley.) FIGURE 110. Typembryo; dorsal view of larva after transformation, h, head; ds, dorsal valve; hi, hinge-line of dorsal valve; p, pedicle. (After Kovalevski.) FIGURE 111. Dorso-ventral longitudinal section based on preceding; showing mantle lobes directed forward, bringing interior shell-secreting surfaces, ds and vs of figure 109, on the exterior, h, head; ds, dorsal valve; hi, dorsal hinge ; vs, ventral valve ; hi', ventral hinge ; p, pedicle. FIGURE 112. Dorsal view of early nepionic shell; showing large posterior opening between valves. (After Kovalevski.) FIGURE 113. Profile of same, ds, dorsal valve; vs, ventral valve; p, pedicle. condition represents the extent of the development of these parts in Meristina rectirostris Hall or Gwynia capsula Jef- 262 STUDIES IN EVOLUTION freys, which lack deltidial plates in the adult shell. The young of other telotremate species, as Magellania flavescens or Terebratulina septentrionalis, agree in the same respect. 114 119 115 -Id 118 120 116 FIGURE 114. Delthyrium of young Rhynchonella, without deltidial plates. FIGURE 115. The same at a later stage, with two triangular deltidial plates. FIGURE 116. The same after completed growth; showing joining of deltidial plates, and limitation of pedicle-opening to ventral beak. FIGUBE 117. Dorsal view of Magellania Jlavescens ; showing completed deltidial plates, del. FIGURE 118. The same; profile, ds, dorsal valve; vs, ventral valve; p, pedicle. FIGURE 119. Dorsal view of umbonal portion of adult Terebratulina septentrionalis, with shell removed by acid; showing slight secondary exten- sion of ventral mantle around pedicle (consequently small deltidial plates are secreted in this species). Mantle areas secreting deltidial plates are shaded. FIGURE 120. Dorsal view of umbonal portion of Magellania Jlavescens, with the shell removed by acid ; showing the complete envelopment of base of pedicle by secondary expansions from ventral mantle, and consequent production of deltidial plates filling delthyrium except at pedicle-opening. See figure 117. An examination of .the animal at this stage shows that the mantle lobes line only the interior of the valves proper. The exposed edges of the mantle are around the peripheries of the valves and also that portion of the ventral mantle DEVELOPMENT OF THE BRACHIOPODA 263 border limiting the deltidial opening and passing along the sides of the pedicle at its base. The ventral mantle grad- ually extends from each side as two prolongations partially covering the opening and enveloping the proximal portion of the pedicle. As this is an extension of the shell-secreting surface of the mantle, there naturally results the formation of two plates within the deltidial area. Their structure is commonly punctate whenever the valves are punctate. These outgrowths or extensions of the mantle into the del- tidial area finally touch and coalesce until, as in M. flavescens, the pedicle emerges through an opening in the ventral mantle, and pari passu the deltidial plates unite and limit the pedicle- opening to the beak of the ventral valve. The latter process has been carefully described by Deslongchamps, 6 Clarke, and the writer, 3 and need not be dwelt on here. Figures 119 and 120 of the beaks of T. septentrionalis and M. flavescens with the shell removed show the relations of the ventral mantle to the pedicle, and the portions which secrete the deltidial plates. ,- The deltidium and delthyrium are often simulated in the growth of the dorsal valve in genera having a high cardinal area in this valve. Orthis, Leptcena, Clitambonites, Spirifer, and Strioklandinia may be cited as examples. They cannot be properly correlated with similar parts in the ventral valve, for their origin is quite different. Primarily, a deltidial opening is for the extrusion of the pedicle, and this belongs properly to the ventral valve. The dorsal fissure is the space between the diverging teeth sockets, and may be filled by the cardinal process, as in Leptcena and Orthis, or it may have in addition a convex plate or chilidium covering it, as in Clitambonites. In Spirifer and Stricklandinia the opening remains unclosed. The true deltidial plates are formed on the side of the pedicle adjacent to the hjnge by extensions of the ventral mantle lobe, and begin as two plates. They are likewise expressive of maturity, and are of secondary development, while the deltidium begins as a single plate in the median 264 STUDIES IN EVOLUTION line, and is eminently a primitive character in the Pro- tremata. From present knowledge of the group it is difficult to offer an explanation for the presence of an anal opening in the Inarticulata and its absence in the recent Artie ulata, as the solution of the question depends upon whether the class is to be considered as progressive or degraded. The dorsal beaks of Amphigenia.) Athyris, Cleiothyris, Atrypa, and RJiyncho- nella are usually notched or perforate. The perforation comes from the union of the crural plates above the floor of the beak leaving a passage through to the apex. A similar opening occurs between the cardinal processes in Strophomena, Stropheodonta, and allied genera, and the chi- lidium may also be furrowed, as in Leptcena rhomboidalis. This character is evidently in no way connected with the pedicle-opening, but points to the existence, in the early artic- ulate genera, of an anal opening dorsal to the axial line, as in the recent Crania. This dorsal foramen was described and fig- ured by King 13 in 1850, Hall 8 in 1860, and by several authors since, and has commonly been termed a visceral foramen. GEhlert 21 suggests that it was probably occupied by the terminal portion of the intestine. The persistence of the foramen seems to indicate an anal opening. In reference to this character and the obsolescence of the eyes the class must be viewed as retrogressive since Paleozoic time. Other features, however, are manifestly progressive; namely, the gradual shortening, through time, of the posterior elements of the animal, as the pedicle, visceral portions, and internal shell structures, and the expansion of the anterior parts, as the shell and brachia. A further advance in specialization is shown in the limita- tion of the pedicle -opening wholly to the ventral valve in the higher rhynchonelloids, athyroids, spiriferoids, and terebratu- loids. The absence of punctse in all the early radicles and their subsequent development in the derived types may also have a similar bearing. DEVELOPMENT OF THE BRACHIOPODA 265 The features and importance of the protegulum have pre- viously been discussed. 1 It is merely noticed here as the embryonic shell of the completed phylembryonic period, for it is the first stage which can be observed among the fossil species, and is the initial point for the discussions of the relations and affinities of recent and fossil forms. Of the protegulum and later stages, there is abundant material avail- able in nearly every family of brachiopods, ranging through their entire geological history. Post-embryonic Stages. In discussing the post-embryonic stages of growth two aspects of development must be clearly differentiated ; (a) the ontogenetical, and (6) the phylogenetical. The ontogeny of a form like Schizocrania may be conveniently divided into the nepionic, neanic, and ephebic periods, and such stages may be clearly defined. The ephebic stage of Schizocrania, however, is like a neanic stage of Orbiculoidea. In other words, Orbiculoidea, in its development, passes through a fSchizocrania-like stage before reaching maturity.* These facts must be viewed from a phylogenetic standpoint. More- over, in the geological history of a group, certain ephebic characters of early species may become accelerated, and pass into the neanic period of later forms, while other characters remain ephebic. Discinisca offers an illustration of this. Its neanic characters agree with Orbiculoidea in the form of the valves and in the pedicle-notch, but the circular or ellip- tical form of the dorsal valve in adult and neanic Orbicu- loidea appears so early in Discinisca that it marks all the nepionic stages. The interpretation of these facts is, of course, very evident, and will be subsequently given in detail. Attention is here called to the statement, that while nepionic, neanic, and ephebic stages represent equal intervals * Attention was called to this fact in a publication preliminary to vol. viii of the Palaeontology of New York, pp. 131, 132, issued February, 1890. Also, the development of the pedicle-opening in Orbiculoidea was fully described. 266 STUDIES IN EVOLUTION in the life of each individual, they do not represent condi- tions of growth, or the possession of characters which always agree, stage for stage, in the species of one family or of different families. Other distinctions to be made whenever possible are (a) whether certain characters (natural or acquired) belong to a species by inheritance, or (6) are mere adaptations to special conditions of environment arising at any time in its history. A clear understanding of the first will lead to the true phylog- eny of a species or genus, but to reach this the characters of the second category must be excluded. Thus in the series of tSchizocrania, Orbiculoidea, and Discinisca, already cited, there is an apparent genetic connection in the facts as stated. The contrary must be the case with shells like Lingula complanata Williams and L. riciniformis Hall, which initiate a holoperipheral * mode of growth in the ephebic period, for this agreement in the method of concrescence with adult Orbiculoidea here appears in the mature stages of this species, and being absent in the early members of the genus cannot therefore be an ancestral character. It is a morphological equivalent, which may or may not be continued in the later species of the series. Whenever features are present which can be referred to an ancestral origin, their elimination can take place only by the process of acceleration of development. On the other hand, there may be secondary characters of dynamical or homo- plastic origin which appear simultaneously or independently in different groups belonging to diverse genetic lines, as the deltidial plates of the Rhynchonellidse, Terebratulidae, and Spiriferidse. Further, many such secondary features may occur anywhere in the geological history of the group, as the high hinge-area of Orthisina, Spirifer, Syringothyris, and Thecidium. These statements are in full accord with what Hyatt has determined in the Cephalopoda, and the applica- tion of such ideas affords a fertile field of research. * 8Aos whole, and irfpupepfia circumference. DEVELOPMENT OF THE BRACHIOPODA 267 Preliminary to a study of the stages of growth observed in the different orders, a simple characteristic example of each will be taken to show the limitations of the post- embryonic periods. Nepionic Period. In brachiopods, as in pelecypods, this period represents the growth of the true shell immediately succeeding the embryonic shell or protegulum, and before the appearance of definite specific characters. In general, the nepionic shells of all groups are marked only by fine concentric lines of growth, and are therefore nearly smooth. Sometimes, however, a few radiating striae or other orna- ments may appear over the nepionic portion, but this is not the prevailing rule. Obolus pulcher Matthew shows a can- cellated nepionic -stage and is one of the most striking excep- tional examples. Plate XII, figure 1, represents the nepionic stage of Glot- tidia albida, drawn from the beak of a well-preserved adult. The shell at this period had a short straight hinge (originally the hinge of the protegulum), with lines representing ante- rior and lateral growth, making the outline broadly ovate. It is divided from the succeeding growth of later stages by a strong varix. The form is suggestive of Obolella, and as this is the early form of growth of many of the Lingulidse and allied families, it is here called the Obolella stage. It is not known that otherwise the characters agree with those of Obolella, but as it is characteristic as well as descriptive the name is used to designate this form of nepionic growth when- ever present. The nepionic stage of Orbiculoidea minuta (figure 4) shows a continuance of the straight-hinged condition after the com- pletion of the embryonic shell, with nearly equal incremental lines. As this agrees with the shell of Paterina [= Iphidea] it is called the Paterina stage. The pedicle emerged freely between the cardinal margins of the valves. It will be shown that both this and the Obolella stage are represented in the nepionic periods of many genera belonging to the Atremata. They may succeed each other in a single species 268 STUDIES IN EVOLUTION or one alone may be present. In case both appear, the Paterina stage is always the first one to be developed. The nepionic stage of Leptcena rhomboidalis (figure 7, Plate XII) is represented by a shell without radii, having a comparatively large pedicle-opening in the ventral valve and a large deltidium. The hinge is not well defined and the shell is discinoid in form. This term is not used to suggest any special affinities with true discinoid genera, as Orbiculoidea or Discinisca. The proper name for this stage is not yet apparent to the writer. The external characters as expressed by both valves are manifestly nearer to Kutor- gina than to any telotremate genus. Until the early forms belonging to the articulate brachiopods, especially to the orthoid and strophomenoid groups, have been thoroughly studied, the interpretation of the nepionic Leptcena rhom- boidalis may be uncertain. It should be noted, however, that the young of Chonetes, Productus, Stropheodonta, Ortho- thetes, Leptcena, Plectambonites, and Strophomena, all have little or no indication of a straight hinge -line, and that the extension of this member takes place during later neanic and ephebic growth. This in itself is significant, but is more marked when taken with the growth-stages shown by some species of Strophomena which have after the protegu- lum a Paterina-like stage, with a straight hinge in the dorsal valve, succeeded by holoperipheral, discinoid, nepionic growth, and finally a renewal of a straight-hinged condition. Thus it has an early straight-hinged form, which is lost during the next stage of growth, and again appears, and is progressively elongated during neanic and ephebic growth. The nepionic stages of Terebratulina septentrionalis (figure 10, Plate XII) represent a decreasing extension of the cardi- nal line from the protegulum, an open delthyrium, the absence of radii, and the introduction of the shell punctse. The crura at this stage, as shown by Morse, are short and stout, and the loop is undeveloped. Neanic Period. During the progress of this period all DEVELOPMENT OF THE BRACHIOPODA 269 the features which reach their complete growth in the adult organism are introduced and progressively developed. Usually they appear in succession, and gradually assume mature conditions. Thus in many species with radiate plica- tions or strise, a few radii appear in early neanic growth, and are added to until the full number is present. Species with deltidial plates develop them in this period. The early stages may offer many points for comparison with the adult, but later stages usually differ little except in size. Figures 2, 5, 8, and 11, Plate XII, represent a neanic stage in each of the four species taken as examples. Others from the same species could be given, but these suffice to show that one or more characteristic adult features have made their appearance. Ephebic Period. The period of complete normal growth, or the maximum of individual perfection. This corresponds to the adult, or mature organism, and is so well understood that no further explanation is necessary. For the sake of completing the series, the .ephebic shells of the species given are represented in figures 3, 6, 9, and 12, Plate XII. Gerontic Period. The variations due to old age may be numerous and complex. As shown by Clarke and the writer, 3 the valves generally become thickened, and, as a consequence, the margins are truncate or varicose, the ver- tical diameter of the shell is increased, the beaks involuted, and the margins of the valves often lose the ornamentation characteristic of the species. The deltidial plates or del- tidium may be resorbed as well as the beaks of the valves. Usually the ephebic characters disappear in inverse order to their introduction. Thus in a normal adult brachiopod hav- ing a plicate shell and deltidial plates, which characters were introduced during the neanic period, the expression of old age will be found in the absorption of the deltidial plates and in the obsolescence of the plications. Large specimens of Terebratella transversa Sowerby often furnish examples of this condition. 270 STUDIES IN EVOLUTION The gerontic development of Bttobites* consists in the obsolescence, in B. various Conrad, of the bilobed form of the shell, thus reverting to an early neanic condition equally characteristic of B. bilobus and B. Verneuiliamis. Another aspect of growth and decline is manifest when the size of individuals and the chronological history of groups are taken into consideration. Each genus and family began with small representatives, and rapidly developed the more radical varieties of structure. Then came the culmination and final reduction in size, with abundance of gerontic and pathologic forms. The oldest known shell with calcareous spires, Zygospira, is a comparatively minute form. Nearly all the types of the sub-order to which this genus belongs (Helicopegmata) appear in the Upper Silurian. Species pre- senting the maximum size belong to the Devonian and Carboniferous. Before the- extinction of the sub-order in the Trias, the individuals are small, and such abnormal genera as Thecospira, Koninckina^ and AmpJiiclina abound. Productus begins with small species (Productella) in the Lower Devo- nian, and in the Carboniferous attains the largest dimen- sions of any known brachiopod (P. giganteus). During the Permian the species have dwindled in size, and the gerontic Strophalosia and Aulosteges are the chief repre- sentatives. The culmination of gerontic growth results in the rever- sion of the animal to its own nepionic period, and is called the paragerontic stage. As this is an extreme condition, it can be found only in certain genera and species which have been developed by a process of accelerated gerontic heredity. If G-wynia * is accepted as a valid genus, it belongs to a pro- nounced paragerontic type. The shell has a small internal plate on each side of the dorsal umbo, evidently the bases of crural plates. King, 14 the author of the genus, states that * Some authors have been disposed to consider this form as the young of a species not yet determined. It has also been referred to Macandrevia cranium, Cistella cistellula, and C. neapolitana. This question cannot be at present deter- mined, although some characters of the shell indicate a mature organism. DEVELOPMENT OF THE BRACHIOPODA 271 the labial appendages are attached directly to the shell, and not to a loop, as in other genera of the family. Cistella may be taken as a representative of paragerontic development among the terebratuloids. The species are smooth or pau- ciplicate, and small; deltidial plates obsolescent, loop more or less undeveloped. In 0. neapolitana the lamellae of the loop are nearly obsolete and are free only near the crura, while the anterior portions are confluent with the valve (Shipley). A slight progression of these reversions would naturally result in a degenerate form like Gwynia, which is, without a calcareous loop; with no surface ornamentation; deltidial plates absent; punctse few and large, all of which features are strictly nepionic. Besides Cistella and Gwynia, other loop-bearing genera present paragerontic features of importance in a natural classification. These consist mainly in their small size; the absence of surface ornaments; the obsolescence of deltidial plates, and the loss of a complete loop supporting the arms. In the Terebratulidaa Kraussina and Platydia may be mentioned as belonging to gerontic types with a paragerontic tendency. Likewise, in other groups, Atretia in the Rhynchonellidse, and Strophalosia and Aulosteges in the Productidas, are examples of paragerontic types. Cistella and G-wynia among the genera of brachiopods, therefore, bear the same relation to the terebratuloids that Baculites among the cephalopods bears to the ammonoids. Synopsis. Protembryo. Ovum and segmented stages before formation of blastula cavity. Mesembryo. Blastosphere. Metembryo. Gastrula. Neoembryo. Trochosphere and cephalula, with posteri- orly directed mantle lobes, and bundles of setae from body segment. Typembryo. Larva with mantle lobes folded anteriorly over head segment. 272 STUDIES IN EVOLUTION Phylembryo. -Bachiopod covered by protegulum, tentacles of arms developed, bundles of setae dehisced, definition of stomach and oesophagus, direct transformation of larval muscles into those corresponding to muscles of adult animal. Deltidium. A single plate developed at an early period by the body and pedicle of animal posterior to dorsal hinge, and later ankylosed to ventral valve. Deltidial Plates. A neanic and adult feature produced by the extensions of the ventral mantle lobe into the delthy- rium. Brachiopoda. Retrogressive in loss of anal opening and eyes, progressive in concentration of posterior elements, expansion of anterior elements, and limitation of pedicle-opening to one valve. Nepionic Period. Young shells before the appearance of distinctive specific characters. Neanic Period. Progressive development of the specific features which reach their complete growth in the adult. Ephebic Period. Normal adult condition. Gerontic Period. Special manifestations of old age in ontog- eny and in phylogeny. Paragerontic Types. Extremes of geratology represented by Cistella, Gwynia, and Atretia. References. 1. Beecher, C. E., 1891. Development of the Brachiopoda. Part I. Introduction. Amer. Jour. Sci. (3), vol. xli, April. 2. 1891. Development of Bilobites. Amer. Jour. Sci. (3), vol. xlii, July. 3. and Clarke, J. M., 1889. The Development of some Silurian Brachiopoda. Mem. N. Y. State Mus., vol. i. No. 1. 4. Brooks, W. K., 1879. The Development of Lingula and the Sys- tematic Position of the Brachiopoda. Johns Hopkins Univ., Chesa- peake Zool. Lab., Sci. Results, Session of 1878. 5. Davidson, T., 1851-1885. A Monograph of the British Fossil Brachiopoda. Pal. Soc., London. 6. Deslongchamps, E., 1862. Note sur le developpement du deltidium chez les brachiopodes articuls. Butt. Soc. Geol. France (2), t. xix. DEVELOPMENT OF THE BRACHIOPODA 273 7. Fewkes, J. W., 1885. On the Larval Forms of Spirorbis borealis, Daudin. American Naturalist, March. 8. Hall, James, 1860. Palceontology of New York, vol. iii. 9. Hyatt, A., 1888. Values in Classification of the Stages of Growth and Decline, with Propositions for a New Nomenclature. Proc. Boston Soc. Nat. Hist., vol. xxiii, March. 10. 1889. Genesis of the Arietidae. Mem. Mus. Comp. Zool., vol. xvi, No. 3. 11. Jackson, R. T., 1890. Phylogeny of the Pelecypoda. The Avicu- lidse and their Allies. Mem. Boston Soc. Nat. Hist., vol. iv, No. viii. 12. Joubin, L., 1886. Recherches sur 1' Anatomic des Brachiopodes Inarticule's. Archiv Zool. Experimentale (2), t. iv. 13. King, W., 1850. A Monograph of the Permian Fossils of England. Pal. Soc., London. 14. King, W., 1859. On Gwynia, Dielasma, and Macandrevia, three new genera of Palliobranchiate Mollusca, one of which has been dredged in Belfast Lough. Proc. Dublin Univ., Zo'61. Bot. Assoc., vol. i. 15. Kovalevski, A. O., 1874. Observations on the Development of Brachiopoda. Proc. Imp. Soc. Amateur Naturalists, etc., held at the University of Moscow, llth year, vol. xiv. 16. Lacaze-Duthiers, H., 1861. I^istoire naturelle des Brachiopodes vivants de la Mediterranee. Ann. Sci. Nat. Zool., t. xv. 17. Morse, E. S., 1873. On the Early Stages of Terebratulina septen- trionalis (Couthouy). Mem. Boston Soc. Nat. Hist., vol. ii. 18. 1873. Embryology of Terebratulina. Mem. Boston Soc. Nat. Hist., vol. ii. 19. 1873. On the Systematic Position of the Brachiopoda. Proc. Boston Soc. Nat. Hist., vol. xv. 20. Miiller, F., 1860. Beschreibung einer Brachiopodenlarve. Archiv Anat. Physiol., Jahrg. 1860. 21. CEhlert, D. P., 1887. Brachiopodes. Manuel de Conchyliologie, Paul Fischer. Appendice. 22. Shipley, A. E., 1883. On the Structure and Development of Argiope. Mittheil. Zool. Station Neapel, Bd. IV. 23. Walcott, C. D., 1888. A Fossil Lingula preserving the Cast of the Peduncle. Proc. U. S. Nat. Mus. 18 274 STUDIES IN EVOLUTION PART III. MORPHOLOGY OF THE BRACHIA* THE diagnostic value of the brachidium, or calcareous arm supports, of brachiopods has long been recognized, and forms one of the chief characters for generic and family sub-division among the Terebratulacea and Spiriferacea. This character fails in all other brachiopods, which have simply fleshy arms, unsupported by calcareous skeletons. There is, however, generally the most obvious analogy and intimate relationship between the arms themselves and the brachidium, so that whenever either structure can be ascertained it furnishes important data aiding in the determination of the systematic position of any genus within a family or order. The growth of the arms, or lophophore, in recent genera may be divided into distinct stages, which often have a direct correlation with other important features of the shell. In many cases it is also possible to infer the form and arrange- ment of the brachia in fossil genera from markings on the interior of the valves and from the calcareous arm supports, and thus to obtain the chronogenetic as well as the morpho- genetic history of these organs. The most detailed accounts of arm development are given by Brooks 5 f for Grlottidia, by Morse n for Terebratulina, and by Kovalevski 10 for Cistella and Thecidea. These results, combined with original observations by the writer * 2 and occasional descriptions of arm structure by Davidson 7 and other authors, are sufficient to include and to interpret prop- erly all the leading varieties of structure. As shown by Brooks 5 the tentacles, or cirri, in Grlottidia originate on the dorsal side of the oral disk. They grow in pairs, one on each side of a central lobe. New tentacles are added between the first pair formed and the median lobe. * Bulletin 87, U. S. Geol. Surv., Chapter IV, 105-112, 1897. t The references to the literature will be found at the end of this chapter. DEVELOPMENT OF THE BRACHIOPODA 275 Thus the cirri farthest removed from the median lobe are the oldest. Tentacles are added rapidly until the first arc is ex- tended to a semi-circle, and then progressively the whole disk becomes surrounded by a circle of these organs. The further introduction of cirri can take place only by the enlargement of the oral disk or through the deformation of the circle by lobes, loops, or extensions. In &lottidia, Lingula, Discinisca, Crania, and Rhynchonella the two points of tentacular in- crease, originally together and on opposite sides of a median lobe, or tentacle, gradually separate, and the further multi- plication of tentacles results in strap-shaped extensions on each side, which finally assume a coiled form, due to the limited space in which they grow. Therefore the arms in adult individuals of these genera have a single cirrated edge, extending from their free extremities to the sides of the oral disk, and, continuing posteriorly, unite on the ven- tral side of the disk behind the mouth. Each cirrated edge in the adult lophophore apparently has two approximate rows of alternating cirri (Hancock 9 ,), but as they were originally a single row in early stages, this appearance is evidently the result of a crowding of the cirri or a crumpling of the edge. Kovalevski 10 has shown that in Cistella the tentacles also originate in pairs on each side of the dorso-median line, with- out a central tentacle or lobe. The same mode of increase has been shown by the writer 2 to be present in Magellania and Terebratalia. In young stages of Cistella, Terebratulina, Magellania, and other terebratuloid genera, as well as in The- cidea, after the circlet of tentacles is complete the two points at which new ones are added do not separate, but remain close together throughout the life of the animal. In this case the cirrated margin is lengthened by means of lobation and loop- ing, and often by the final growth of a single, median, coiled arm, cirrated on both margins. G-wynia illustrates the com- pleted circle of tentacles about the mouth. Adult Cistella shows an advance in having the anterior margin of the lopho- phore introverted, making it bilobed. Megathyris is slightly 276 STUDIES IN EVOLUTION more complicated by two additional lobes. This simple method of increase is further elaborated in the Thecidiidse. In the higher genera, especially among the Terebratulidse, the maximum is reached by means of a median, unpaired, coiled arm, as in Magellania and Terebratulina. The development of the different types and varieties of arm structure is presented in the accompanying figures (121- 125), which are necessarily somewhat diagrammatic in order to show the features clearly, but the essential structure can be readily verified from consultation of the works cited or from a study of actual specimens. In the case of fossil forms, such as Dielasma, the Atrypidse, and Athyridse, the brach- ial supports have sufficient analogy with the arm structures of Terebratulina and Rhynchonella to warrant their interpreta- tion as given. Also, the spiral impressions on the valves of Davidsonia, and those occasionally present in Leptcena and Producing clearly point to the possession of coiled arms by these genera. Classification of Brachial Structures. From what has already been shown it is seen that the various types of lophophore admit of a simple classification into stages and groups. It is proposed to give to these distinctive names, which may be used with facility in making comparisons and correlations. They may be found useful, also, in designating the kind of brachial complexity attained in any genus the arm structure of which can be determined, thus helping to fix its place in a genetic scale. It should be emphasized, however, that the form and complexity of the cirrated margin of the lophophore can have a taxonomic value only within comparatively narrow limits. This at once be- comes evident when the arms of Lingula, Discinisca, Crania, Rhynchonella, and all the Spiriferacea are considered. Each has spiral arms, which were probably developed through similar changes of form, and yet each is genetically distinct, DEVELOPMENT OF THE BRACHIOPODA 277 as shown by all the other leading characters. But when this classification of arm structures is applied within a family or genus, or even when made the basis of comparison among some closely related families, it is sometimes possible to reach very satisfactory conclusions relating to the systematic position of various forms. Leiolophus Stage. It is hardly necessary to direct attention to the embryonic brachial structure before the growth of any of the tentacles, or cirri, on the edge of the lophophore, while the animal is in the typembryonic stage. For the sake of designating all the stages, this may be called the leiolophus stage, though it has 110 special significance beyond indicating the beginning of the lophophore. TaxolopJius Stage. The first stage in which a true brachial structure is mani- fest is an early larval form, often the protegulum stage, when the tentacular portion of the lophophore is a simple arc or crescent. This may be called the taxolophus. The tenta- cles are few in number, and increase takes place on each side of the median line, dorsally, in front of the mouth. In figures 121, a, e, 122, a, /, 124, a, this character is clearly shown. The tentacles at the ends of the arc are the oldest, and new ones are being formed in the middle portion. In Thecidea, Cistella, and Magellania the tentacles of the taxo- lophus are centripetal, due to the edge of the lophophore being near the margin of the shell; while in Terelratulina, Disci- nisca, and Lingula they are centrifugal, due to the smaller and central lophophore. So far as known, there is no adult living form which has the taxolophian brachial structure. It may have been pres- ent in adult Iphidea of the Cambrian. 278 STUDIES IN EVOLUTION Trocholophus Stage. By the continual addition of new cirri and the pushing back of the old ones, the fringed margin of the lophophore passes from a crescentic to a circular form, thus making a complete ring about the mouth. This may be termed the trocholophus stage. It appears in the late larval and early adolescent stages of Thecidea (figure 121, 6), Cistella (figure 121,/), Magellania and Terabratalia (figure 122, 5), Terebratu- lina (figure 122, #), Grlottidia (figure 124, 5), and Discinisca, and, like the former stages, is undoubtedly common to all brachiopods, except, perhaps, Iphidea. G-wynia is an adult living representative of this stage, and never develops any higher type of brachial structure. Dy&- colia also belongs here, since it has a discoid lophophore surrounded by a marginal fringe of tentacles (Fischer and (Ehlert 8 ). It is possibly a little more advanced than G-wynia, as it has a slight median anterior notch, suggesting the begin- ning of the bilobed structure of the next higher type. The absence of septum, hinge-plate, and dental plates are other primitive characters belonging to Dyscolia. Schizolophus Stage. After the completion of the trocholophus stage in all brachi- opods, except such simple forms as Gwynia and Dyscolia, no further increase in the cirrated edge of the lophophore can occur without some deformation of the circle. This is first accomplished by an introversion of the anterior median edge, thus dividing the lophophore into two lobes, and suggesting the name schizolophus for this type. (See figures 121, c, g, 122, c, h, 124, c.) Several brachiopods retain the schizolophian brachia as an adult character. Of these, Cistella is perhaps the best example, as it agrees exactly with an early stage of arm structure among the Terebratellidse, which has been called the cistelliform stage (figure 122, c?). Terelratulina (figure DEVELOPMENT OF THE BRACHIOPODA 279 122, A), G-lottidia (figure 124, c\ and other higher forms also have corresponding schizolophian stages, but are without the median septum. Lacazella mediterranea presents a similar larval structure, and in L. Barretti it is retained to maturity. The fossil genera Davidsonella and Thecidella of the The- cidiidsB, and Zellania of the Terebratellida3, never developed beyond the schizolophus -stage, and they must therefore be considered as quite primitive genera in their respective families. 121 v^ $ Taxolophus. Trocholophus. Schizolophus. Ptycholophus. FIGURE 121. Stages of growth of the lophophore in Thecidea, Cistella, and Megaihyris. a, b, c, d, stages in the growth of the lophophore in Thecidea (Lacazella) mediterranea. Enlarged, (a-c, after Kovalevski; d, after Lacaze- Duthiers.) e, f t early stages of lophophore of Cistella neapolitana. Enlarged. (After Kovalevski.) g, adult lophophore of Cistella (C. cistellula). Enlarged. (After Davidson.) h, labial appendages of Megaihyris decollata. Enlarged. (After Davidson.) From this point the further development and complication of arm structure proceeds in three distinct diverging lines, producing the three characteristic types of brachia of all the higher brachiopods, as exemplified in Thecidea^ Terebratulina, and Rhynchonella. 280 STUDIES IN EVOLUTION Ptycholophus Stage. The simplest of the types of brachia just cited is developed out of the schizolophus by the additional lobation, or loop- ing, of the primary lobes, making a structure which may be Taxolophus. Trocholophus. Schizolophus. Zugolophus. Plectolophus. FIGURE 122. Stages of growth of the lophophore in the Terebratellidje and Terebratulidae. a, 6, c, d, e, five stages in the development of the lophophore in the Terebratellidae. a-d, Terebratalia obsoleta. Enlarged. (After Beecher. 2 ) e, Magellania kerguelenensis. Natural size. (After Davidson. 7 ) /, g, h, i,j, develop- ment of lophophore in the Terebratulidae. /-, early stages in Terebratulina septentrionalis. Enlarged. (After Morse. 11 ) /, adult Terebratulina cancellata. (After Davidson.7) called the ptycholophus. Megathyris and Lacazella mediter- ranea both have four lobes (figure 2, d, ); Thecidea radiata has six; T. vermicularis and Eudesella may ale, eight; E. digitata, ten; Pterophloios and Oldhamina, about twenty. Lobation in some (Ttiecided) is produced by the forking or DEVELOPMENT OF THE BRACHIOPODA 281 branching of the median septum; in others (Pterophloios) the septum remains simple, while the lateral borders of the lophophore are lobed. Zugolophus and Plectolophus Stages. All the higher Terebratulacea reach the final growth of the lophophore through an intermediate stage which from its form may be called the zugolophus (figure 122, d, i). Eucala- this and Platidia (? Tropidoleptus) are apparently adult rep- resentatives of this stage, while Kraussina and probably Bouchardia are slightly more advanced by the growth of a short median, coiled arm, and lead to the next higher, or plectolophus, stage, in which there is a well-developed spiral arm with a fringe of cirri on each edge (figure 122, e, /). A long loop pointed in front, like Rensselceria and Centra- nella, could not have supported a median arm, as the pallial cavity is thus fully occupied, and the development of the brachidium in the Terebratellidse shows that the central space between the branches of the loop is to accommodate such an organ. The same is doubtless true of Dielasma, which first has a Centronetta-\ike loop, and through the subsequent resorption of the anterior portion the ascending branches are formed and space allowed for the median arm (figure 123, a-d). In a spire-bearing genus like Zygospira this is more obvious, for here the transverse process or jugum is clearly the result of the growth and resorption of the cen- tronelliform loop to admit the spiralia. 123 FIGURE 123. Metamorphoses of the brachidium in Dielasma turgidum. Enlarged. (After Beecher and Schuchert.) The calcareous loop in Terebratulina and Liothyrina is only a posterior basal support, and does not repeat the out- 282 STUDIES IN EVOLUTION line of the cirrated margin of the lophophore, exclusive of the arm. Therefore it is impossible in these and closely allied genera to infer the stage of development of the lopho- phore from the loop alone. Dyscolia is an excellent example, 124 _ since the loop is the same as in Terebratulina ; but the lophophores are quite distinct in each, the former being of the trocholophus type and the latter belonging to the Trocholophus. plectolophus. Taxolophus. Spirolophus Stage. The last type to be noticed is the one in which there are two Schizolophus. -T -I i separate coiled arms, each with a row of cirri on one edge only (fig- ure 124, d, e). It embraces the greater part of the families of brach- Spirolophus. io pods in the orders Telotremata and Protremata, and includes all the living species in the orders Atremata and Neotremata. In the early stages of develop- ment of the spiral lophophore there is an agreement with the early FIGURE 124. Early stages stages of the families alreadv no- of lophophore of Glottidia, and , i -, ., , , , adult brachia in Lingula and tlCed ' and the taxolophus, trochol- Hemithyris. a, b, c, early stages Ophus, and Schizolophus Stages may Brooks.) d, adult brachia in The separation and growth of the Lingula. (After Woodward.) sp i ra l arms seem to be due to the e, adult brachia in Hemithyris . -. . psittacea. (After Hancock.) widening or expansion of the me- dian lobe or tentacle, on each side of which is the formative tissue for new cirri. This is very apparent in the young Ducinisca described by Muller, 12 and the G-lottidia described by Brooks. 6 The brachidium in Zygospira passes through a series of DEVELOPMENT OF THE BRACHIOPODA 283 changes which have been described in detail elsewhere.* These metamorphoses are of great assistance in understand- ing the development and comparative morphology of this feature in other groups of the Spiriferacea. The earliest stage observed (figure 125, a) has the form of a simple tere- bratuloid loop, which, from its resemblance to Centronella, was called the centronelliform stage. Since approximately this form of brachidium is also characteristic of the young of recent terebratuloids, it may be taken in Zygospira as indicative of the trocholophus stage of brachial development. With this as a starting-point for comparison, the further correlation of the succeeding stages is very simple. The first resorption of the end of the loop in Zygospira produced a schizolophus condition, and further resorption carried the brachidium to a stage closely resembling Dielasma (figure 125, 6). The dielasmatiform stage has already been explained as due to the requirements of space for the growth of the coiled brachia. Next, the initial calcification of the spiral arms resulted in i^ie extension of the descending branches beyond the jugum (figure 125, f combined with previous observations by Friele n and Des- longchamps, 7 furnish material which suggests a natural grouping of the terebratuloids. The present knowledge is incomplete in some details, especially as regards the fossil genera, yet enough is available to simplify the arrangement of the leading terebratuloid types, and to show their common relationships. By far the best classifications have been those proposed by Dall 3 in 1870, and by Deslongchamps 7 in 1884. Only in the light of recent discoveries is it possible to offer a new arrangement of the genera. The sub-order Ancylobrachia, proposed by Gray 12 in 1848, includes, with some emendations, all the genera currently known as terebratuloids. Taking Gray's name for the entire group, since it has priority over Kampylopegmata, Waagen, 16 1883, it is found to comprise two distinct types of brachial structure, each with a separate genetic history. It is here proposed to recognize these two types as of family impor- tance, according to the interpretation of family characters given by Agassiz. 1 The TerebratulidoB. In the first family, the Terebratulidse, the loop is always free and may be long or short. It is developed by the growth * Trans. Conn. Acad. ScL, IX, 376-391, 395-398, pis. i, ii, 1893. t The works referred to by numbers are cited in full in the list appended. An excellent summary and review of Fischer and (Ehlert's papers, 8 9 . 10 by Miss Agues Crane, 2 appeared in the January number of Natural Science, 1893. FAMILIES OF LOOP-BEARING BRACHIOPODA 291 of two lamellae, or descending branches, from the points of the crura, uniting in the median line. The central portion may be narrow or medially expanded. In some genera, re- curved ascending branches are produced by the partial resorp- tion of the broad band or plate forming the connection between the descending branches. The cirri in early stages of the animal are centrifugal or directed outward. The growth of the loop in Terebratulina has been illustrated by Morse. 15 Terebratula (Liothyrina) and Terebratulina may be selected as best representing the Terebratulidse ; for Dyscolia, Agul- hasia, and Eucalathis do not represent the highest develop- ment of the family type, but must be regarded as degraded forms. Among fossil genera Cryptonella, Megalanteris, Die- lasma, Centronella, Rensselceria, Stringocephalus, and some others, probably belong here. The following sub-families can be recognized : (1) the Centronellinse, (2) Stringocephalinse, (3) Terebratulinse, and (4) Dyscoliinse. The adult arm structure in Dyscolia is homologous with early larval features in Terebratulina; also the cirri are centrifugal or directed outward, as in early stages of Terebratulina^ and not cen- tripetal as in larval Magellania. The TerebratellidcB. The loop in the second family, for which the name Tere- bratellidse is retained, undergoes a series of metamorphoses while attached to a dorsal septum during the larval and im- mature stages of the animal, and in the higher forms results in a loop of secondary growth much like the primary loop of some of the early genera of the Terebratulidse. The cirri in larval stages of the animal are centripetal or directed inwardly. In one division of the Terebratellidse the stages of growth may be correlated with the adult loops in the genera Gwynia, Cistella, Platidia, Ismenia, Muhlfeldtia, Tercbratalia,* and Dallina;^ while in another division a quite different series * Type Terebratula transversa G. B. Sowerby. t Type Terebratula septigera Love'n. 292 STUDIES IN EVOLUTION of transformations takes place. These have been termed, by Fischer and (Ehlert, 8 the prcemagadiform, magadiform, maga- selliform, terebratelliform, and magellaniform stages, from their resemblance to the loops of the genera suggesting these names. The prcemagadiform stage is here divided into the bouchardiform and megerliniform stages. To these may be added the earlier larval stages resembling G-wynia and Cis- tella, as in the previous group, and showing a parallel development in the first two stages. These two groups of the Terebratellidae usually have been considered as part of the family Terebratulidae, although King, 14 in 1850, proposed the name Terebratellidae to include Terebratella, Muhlfeldtia, and Ismenia, on account of the attachment of the loop to the septum of the dorsal valve in these genera. Friele n and Deslongchamps 7 next showed that Macandrevia cranium and Dallina septigera passed through a series of changes in which the loop was united to a septum in all but the last stage. This completed loop in Macandrevia, composed of two descending and ascending lamellae, was be- lieved to be homologous with the loop of Terebratulina and Liothyrina, and the family proposed by King fell into disuse. It can now be shown, however, that the loop of Macandrevia is made up of a primary portion corresponding to the entire loop of Liothyrina, and a secondary part which has no equiv- alent in the calcified lamellae of Liothyrina or Terebratulina, but in them is represented in the fleshy portion of the arms, as previously recognized by Hancock. 13 The loop in Terebratulina is equivalent to the descending lamellae in Terebratella, from the crural points down to and including the bands connecting with the septum. In Magel- lania and Macandrevia the connecting bands of Terebratella are represented, except in old specimens, by slight projec- tions from the descending branches, and in these genera, therefore, the primary loop is incomplete.* The true rela- * The prongs or points below the ends of the crura on the primary lamellae in Spirifer also represent portions of a loop. More close analogy is seen in later forms of Atrypa having a disunited loop. FAMILIES OF LOOP-BEARING BRACHIOPODA 293 tions and homologies of these parts can best be shown in a series of figures. Plate XIV, figures Ci, Di, represent the loop in a young Macandrevia cranium in the so-called platidiform stage, show- ing a complete primary loop and the beginning of a secondary loop in the middle, on top of the septum. A later stage of the same species (Plate XIV, figure GI) has the structure of Terebratalia. The descending lamellae and the median V-shaped plate correspond to the primary loop, while the secondary loop or posteriorly recurved portion has greatly increased in size. A later stage, nearly complete (Plate XXIV, figure 1), shows two points (p) on the descending lamellae, which are remnants of the connecting band in pre- vious stages. The parts homologous with the loop of the first stage and with the loop of Terebratulina are shaded. Greater emphasis is expressed by figures 2, 3, Plate XXIV, where the cirrated brachia and calcareous supports are both represented in the genera Terebratulina and Magellania. It is readily seen that the arm structure is the same in both, but that the calcareous loops which are darkly shaded are very different in form. The family Terebratellidae should therefore be reinstated on the evidence here given. The development of Terebra- tella may be reviewed for the leading characteristics of one division of the family. The type is Terebratella chiliensis Broderip, sp. = T. dorsata Gmelin, sp., from the Straits of Magellan. Fischer and CEhlert 8 have described in detail the development of the loop in this form. Their researches also include Magellania venosa Solander, sp., which was found to pass through all the stages of Terebratella dorsata, and after losing the processes connecting the primary lamellae with the septum finally results in adult Magellania. MagellaniincB. The first stage described by these authors (Plate XIV, figure B) showed only a septum anterior to the middle of the 294 STUDIES IN EVOLUTION dorsal valve.* The next stage was called the prcemagadiform stage (Plate XIV, figures Ca, -Da), but it may well be divided into two stages, which correspond in structure to adult Bouchardia and Megerlina. The bouchardiform stage (figure Ca) has a high quadrangular septum in the dorsal valve, and on the posterior distal angle there is a small circle, or calca- reous ring. The crura are present, but the primary lamellae have not yet appeared. In the next stage, the megerliniform (figures Da, Da'), the ring has increased in size, and below, on the septum, have appeared two projections or points, which are the beginnings of the descending primary branches. The subsequent, or magadiform, stage (Plate XIV, figure Ea) shows the completion of the descending branches to form the primary loop, and also the enlargement of the secondary loop or ring. During further growth the primary and secondary loops approach each other on the septum, then coalesce and make the magaselliform condition represented in figure Fa. The ventrally projecting, free portion of the septum next is absorbed, and the branches of the loop become attenuated, but still the descending branches remain connected with the septum, and thus the terebratelliform stage is completed (Plate XIV, figure Ga). Magellania venosa, after passing through all the stages described, including the terebratelliform, loses the connecting bands, and develops into the final magellaniform type of structure (Plate XIV, figure Ha). Moreover, Magellania lenticularis, M. flavescens, Terebratella cruenta, and T. rubi- cunda, as far as observed, correspond closely in their develop- ment with the morphogeny of M. venosa. A fact of importance noticed by Fischer and CEhlert 8 is that these species are confined to the southern hemisphere. The other austral types of terebratuloids, exclusive of the genera of Terebratulidas, as here restricted, are Magasella (M. Cumingi), Kraussina, Megerlina, and Bouchardia. In * An earlier gwyniform stage has been observed by the writer in a young example of Magellania Jlavescens. FAMILIES OF LOOP-BEARING BRACHIOPODA 295 their brachial supports these all approximate early stages of the higher genera Magellania and Terebratella. They must be regarded as arrested and degraded forms. The brachial supports in Kraussina and Bouchardia are merely portions of the ascending branches, or secondary loop, on the septum, without any traces of the descending branches, or primary lamellae. These genera may be compared with the bouchardiform stage of Terebratella dorsata. One grade higher is exhibited in Megerlina (type M. Lamarckiana David- son) in which there is added to the Kraussina structure two processes apparently homologous with the points belonging to the descending branches appearing on the septum in the megerliniform stage of T. dorsata. These atavistic genera are all austral in their distribution, but not strictly polar, occurring as they do off the coasts of South Africa, Brazil, Australia, St. Paul's Island, etc. In reviewing this group of genera, it is seen that the high- est member of the series is Magellania, which reaches its maximum development in size and number of species in antarctic seas. The next genus below, Terebratella, ranges still further toward the equator, while the atavistic types Kraussina, Megerlina, and Bouchardia do not occur in polar regions, but are nevertheless austral in their distribution. Dallinince. The northern hemisphere furnishes a series of genera and species, which, passing through a different and distinct series of loop metamorphoses, attains in the higher members the same result as those of the southern fauna, constituting a case of exact parallel development. Thus the northern Ma- candrevia cranium, Dallina septigera, D. Raphaelis, D. G-rayi, Terebratalia transversa, T. coreanica, T. spitzbergensis, and T. frontalis are very similar in the adult characters of the loop to the southern Magellania venosa, M. kerguelenensis, M. Wyvillii, M. flavescens, M. lenticularis, Terebratella dor- sata, T. cruenta, and T. rubicunda. It is only when their development is examined that a difference is manifest. 296 STUDIES IN EVOLUTION By observing the stages of development in the austral and boreal terebratellids, it is seen that both start from a common larval stage, and divergence into two lines begins in the first adolescent stages, so that the series of metamorphoses in each is quite distinct nearly to the end. This in itself might not require that the austral and boreal species should be referred to different genera and placed in different sub-fam- ilies ; but when it is found that all the other southern genera of the Terebratellidse represent arrested and degraded stages in the development of a southern Terebratella or Magellania, and that the northern genera represent similar stages in the development of a northern high type, such a separation neces- sarily follows.* Moreover, these stages have a more profound significance, as several of them in both regions represent established genera now extinct. A feature which may be of service in distinguishing adult recent shells is, that the Dallininas have small cardinal proc- esses, and the interior of the dorsal beak is usually grooved to the apex, while in Magellaniinse there is a well-developed projecting cardinal process often filling the cavity of the beak. The lower genera can be readily determined by the characters of the loop and by the median septum, which is generally low in the Dallininse and projecting above the loop in the Magellaniinse. With these considerations in mind, the metamorphoses and relations of the northern Terebratellidse may be described. There are two finished types of northern genera, which are * Platidia seems to be an exception in the distribution of the northern genera, as it has been recorded from Marion Island, in the southern Indian Ocean. The northern forms referred to Mayasella are without the characteristic high septum of M. Cumingi, and appear to be stages of development of a higher northern form. In Fischer's "Manuel de Conchyliologie," p. 1246, CEhlert,iu discussing the geographical distribution of brachiopods, says : " Parmi les Brachiopods il en est, dont la distribution est en rapport avec la temperature re"gionale ; c'est ainsi qu'un certain nombre d'especes sont particulieres aux mers qui avoisinent les poles, chaque hemisphere ayant ses formes speciales qui lui appartiennent en propre, a 1'exception de Terebratulina caput-serpentis, var. septentrional is, qui se trouve k la fois dans 1'hemisphere austral et dans I'hemSsphere bore'al." FAMILIES OF LOOP-BEARING BRACHIOPODA 297 taken as characteristic examples. One is the Macandrevia cranium Miiller, and the other has been called Magellania (Waldheimia) septigera Love*n. In the light of the geo- graphic, genetic, and ontogenetic facts, the application of the law of morphogenesis necessitates a new generic name for the second. Magellania cannot be retained, as the type is M. venosa from Tierra del Fuego, and therefore belongs to the southern line having a different series of metamorphoses. Neither can it be referred to Macandrevia, on account of its well-developed septum at maturity ; nor to Endesia (type E. cardium Lamarck, from the Jurassic), since that genus has strong dental plates in the ventral valve, divid- ing the cavity of the beak into three chambers. Waagen 16 shows that these features septal and dental plates are entitled, in the terebratuloids, to rank as generic characters. The name Dallina, nov. gen., is therefore proposed, to include shells of the type of Terebratula septigera Love*n; as Dallina Raphaelis Ball, sp., D. Crrayi Davidson, sp., and D. floridana Pourtales, sp. The genus is given in honor of William H. Dall, whose name has long been intimately asso- ciated with the best work on recent Brachiopoda. There still remain the northern species heretofore referred to Terebratella, which differ from true Terebraiella (type T. dorsata) in the same manner and degree as Dallina from Magellania. These also require a special designation, and the name Terebratalia, nov. gen., is proposed, based on Tere- bratula transversa G. B. Sowerby, as the type. The earliest stages of development in the Dallina and Terebratalia branch of the Terebratellidse have been observed by the writer in T. transversa Sowerby and T. obsoleta Dall. 4 * They represent first a shell without a septum in the dorsal valve, and without calcified supports to the brachia (Plate XIV, figure A). The structure just before the appearance * Originally described as Terebratella occidentalis, var. obsoleta, by Dall, but now considered by him as a distinct species. The complete development of the brachial supports in this species is shown in paper No. 8 of this series. 298 STUDIES IN EVOLUTION of the septum is the same as that described in G-wynia by King. The brachia form a slender fleshy ellipse or circle, resting in front on the floor of the interior of the dorsal valve, with the tentacles or cirri centripetal or directed inward, as in an early stage of Cistella. After this gwyni- form stage the growth of the septum inflects the circlet of tentacles, producing a condition identical with that in adult Cistella (Plate XIV, figure B). It is therefore called the cistelliform stage. The succeeding transformations in Dallina septigera and Macandrevia cranium have been fully described by Friele. 11 These species, with Terebratalia obsoleta Dall, sp., make three typical northern forms whose development has been observed. They agree in every essential detail, and may be described in general terms. The first stage described by Friele (Plate XIV, figures Ci, Di), showed the growth of the descending lamellae, their union with the septum, and the appearance of a small ring on the top of the septum, which is the beginning of the ascending branches, or secondary loop. This condi- tion was correlated with the genus Platidia, by Deslong- champs, 7 and was called the platidiform stage. It has also been called the centronelliform stage by Fischer and QEhlert, 8 but, as Centronella is not known to have a septum supporting the loop, the name is not adopted here. The lower anterior part of the secondary loop begins to divide very early (Plate XIV, figure Di), and, at the same time, the ends of the descending branches broaden and approach the top of the septum, being thus in juxtaposition to the ascending branches (as in figure Ei), called the ismeni- form stage. Lacunse are then produced by resorption in the broad plates forming the ascending branches, and the struc- ture of the supports at this time (figure Fi), resembles that in adult Muhlfeldtia sanguinea and M. truncata (figure F3), in which the secondary loop is still attached to the septum. This stage is here termed the muhlfeldtiform stage. A further broadening of the loop and completion of the structures already outlined, with the recession of the secondary connect- FAMILIES OF LOOP-BEARING BRACHIOPODA 299 ing bands from the septum, result in Laqueus (figures G4, Gs), which has connecting bands from the ascending to the descending lamellae and from the latter to the septum.* The absorption of the connecting bands from the ascend- ing branches completes the Terebratalia stage (Plate XIV, figures Gi, Ga), in Macandrevia and Dallina, and is the adult condition in Terebratalia transversa^ T. obsoleta, T. frontalis, and T. coreanica (figure G3).f Finally, the resorption of the connecting bands from the descending branches produces the Dallina structure, and the further resorption of the septum terminates the series in Macandrevia (Plate XIV, figure Hi). Comparisons and Homologies. Thus the genera of the Terebratellidse begin their larval development in a form like Gwynia, having no calcified brachial supports, and with a simple circle of centripetally directed tentacles. Then by the growth of a septum in the middle of the dorsal valve, a cistelliform stage is reached. From this point divergence begins, and there is one series of transformations resulting in Macandrevia, and another terminating in Magellania, the mature loops in both groups being practically alike. Macandrevia and Dallina are mor- phically equivalent to MageUania, and Terebratalia is also in exact parallelism with Terebratella. A more graphic presentation of the development and rela- tions of the genera is shown on Plate XIV, in which the stages of growth of Magellania and Macandrevia are represented on two ontogenetic lines. Outside are placed other species and genera, with their known stages of growth so arranged * The name Megerlina Jeffrey si was given to a stage of Laqueus californica from its having a structure like Megerlina (= Muhlfeldtid) truncata, thus indi- cating clearly the close relationship of these genera. t T. spitzbergensis and T. Marice, from the unfinished appearance of their brachial supports, possibly will be found to belong to a higher member of the series ; for example, Dallina. 300 STUDIES IN EVOLUTION that their equivalent stages fall into parallel lines with those of Magellania and Macandrevia* The line begins in an early larval stage (Plate XIV, figure A), in which there is a simple circlet of tentacles without a calcined loop, a structure comparable in every respect with Crwynia (figure Aa). The next stage (Plate XIV, figure B) shows the growth of a septum inflecting the line of tentacles, and producing an arrangement of parts similar to Cistella, although the loop is not calcined. The completion of this structure results in Cistella (figure Bi), and specimens having the characters presented by figures B, Bi, are referred to the cistelliform stage. A fossil representative of this type is Zellania, from the Jurassic (figure Ba). Megathyris (figure B2) offers nearly the same structure as Cistella, but the growth of two lateral septa or projections has produced two additional inflections in the loop. This completes the line of development in the MegathyrinsD. Next, considering the Dallininse in their ontogeny and morphology, it is found that after passing through the gwyniform and cistelliform stages (Plate XIV, figures A, B) a form like Platidia is reached (figure Ci), of which Platidia is the living adult representative (figure O). The platidiform stage is shown in Macandrevia cranium (figures Ci, Di) ; Dallina septigera (figure D2) ; D. floridana (figure C3) ; and Muhlfeldtia sanguinea (figures 2, Ds). The growth of the secondary loop on the septum and the subsequent partial resorption produces a structure (1) like that in the fossil genus Ismenia, and (2) one identical with that of adult Muhlfeldtia (^M. truncata and M. sanguinea). In Plate XIV, figures Ei-E5 show the ismeniform stage of Macandrevia, Dallina, and Muhlfeldtia, as well as the final con- dition of Ismenia,^ and figures Fi-F4 represent the muhlfeldti- * The illustrations on Plate XIV are taken from Davidson, 5 - 6 Fischer and CEhlert, 8 Deslongchamps, 7 and Friele, 11 with original drawings by the writer. t Figures Fs and F 6 , Plate XIV, are from Davidson. 5 They are Jurassic species, and were referred to Terebratella (T. furcata Sowerby, figure Fs, and FAMILIES OF LOOP-BEARING BRACHIOPODA 301 form stage of Macandrevia, Dallina, Laqueus, and the adult structure of Muhlfeldtia sanguined. After this, the union of the primary and secondary loops and their removal from the septum to which they remain attached only by connecting processes form a structure like that in Laqueus (figures G*, Gs), and the resorption of the connecting bands from the ascending branches of the loop completes the terebrataliform stage of Macandrevia and Dallina, as shown in Plate XIV, figures Gi, G2. Terebratalia is the present fixed genus of this type of structure (figure Gs), and Trigonosemus (figure G5), is a Cretaceous representative. Finally, by the resorp- tion of the bands of the terebrataliform stage, the structure of the highest genera, Macandrevia and Dallina, is reached (figures Hi-H5). The first stage after the cistelliform in the Magellan iinse, the austral branch of the Terebratellidse, is represented for Terebratella dorsata, in Plate XIY, figure Ca. Kraussina (figures Cb, Cc) has a simple fork or V-shaped process on the septum, which apparently represents an incomplete secondary loop. The relations of Bouchardia (figure Ccf) to this bouchardiform stage of Terebratella are more evident. After this stage the beginnings of the primary loop, or descending branches, appear as two projections on each side of the septum (figure Da'). Megerlina (figure D5) shows this ad- vance over Kraussina. The completion of the descending branches in the next, or magadiform, stage is represented for Terebratella dorsata, in Plate XIV, figure Ea ; T. cruenta, figure EC ; T. rubicunda, figure E<# ; Neothyris lenticularis, figure E> / Magasella Cum- ingi, figure E/. The Cretaceous equivalent, Magas, is shown in figure Ee. In all these forms the septum projects above the descending lamellae nearly to the ventral valve. T. Buckmani Moore, figure Fe). A strict interpretation of that genus based upon T. dorsata, the type, excludes these species, which agree with the definition of Ismcnia in that the ascending and descending branches are attached directly to the septum. They may be, however, stages of growth of higher forms. 302 STUDIES IN EVOLUTION After the magadiform stage the descending and ascend- ing branches approach and unite, and at the same time there is a narrowing of the latter (Plate XIV, figures Fa-Fe). Magasella Cumingi Davidson seems to be the only permanent adult representative of this structure which has yet been found. The further narrowing of the lamellae, broadening of the loop, and absorption of the free portion of the septum, result in the terebratelliform structure (Plate XIV, figures Ga-Gd), comparable directly with figures Gi-G6 of the Dallininae or boreal genera. Also, as in the Dallininse, the disappearance of the connecting bands completes the magellaniform stage, and terminates the series (figures Ha-He). The stages of growth of the genera belonging to the three sub-families of the Terebratellidse, the Megathyrinse, Dal- lininse, and Magellaniinse, are further correlated in the accom- panying tables. It must be understood, of course, that the larval and immature stages have not been observed in all the genera, but from the known ontogeny of several of the lower and higher forms, and from evident homologies of structure, such stages may be inferred. Morphogeny from Gwynia to Megathyms. Periods. Stages. Stages. Stages. Larval Adolescent Mature gwyniform'? gwyniform Gwynia gwyniform cistelliform Cistella gwyniform cistelliform Megathyris The simplest genus, Gwynia, as far as known, passes through no metamorphoses, and has the same structure throughout the adolescent period, up to and including the mature condition. In the ontogeny of Cistella the gwyni- form stage through acceleration has become a larval condi- tion. In Platidia the cistelliform, structure is accelerated to the immature period, and in Ismenia (representing an ismeniform type of structure in the higher genera), the gwyni- FAMILIES OF LOOP-BEARING BRACHIOPODA 303 i gwyniform cistelliform IIii ill! ts s E o - II ill 'H I & SJC'S r p, _o g & S 3 ! ^3 II 12 a 1 &1 "S.1 s g g g I ll 2 e '= II bfi'o 1 S ^H 1 c I u s 1 I bo .22 o * g a - J^ B, & 3 | , 'o | 3 < 1 *, llgll .2 II i $3 f S.S.&S 1 fe to g