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 if 
 
 
UNIVERSITY OF TORONTO 
 STUDIES 
 
 BIOLOGICAL SERIES. 
 
 No. V 
 
 ON THE IDENTIFICATION OF MECKELIAN AND 
 
 MYLOHYOID GROOVES IN THE JAWS OF 
 
 MESOZOIC AND RECENT MAMMALIA, 
 
 BY B. ARTHUR BENSLEY. 
 
 
 THE UNIVERSITY LIBRARY: PUBLISHED BY 
 THE LlBiMRIAN, 1902 
 
oMMin-KK oi m.\na(;i<:mi:ni" 
 
 if 
 
 Chairman J ami,. Loim.on. 1.1.1)., I'rc-M.lnt ..f the 
 I'niviT^ity. 
 
 I'KOFKSSOR W. I. AlKWVl.HK. I'll, 1 >. 
 HrOKKSXiR I'LiHAM ElM.AK, Pll. D. 
 PUINCII'AI J. ('.ALliKAITH. M.A. 
 
 l'R(,ri;s.oK R. Ram>av \Vm.,iiT, M.A., H.Sc 
 Pkoikssok Gkiikck M. Wkon.;, MA. 
 Gemral Rd,U,r . H H. LxwroN, I '.A. L,l.r:ir<an ot the 
 University. 
 
ON THl IPENTIFICATION OF MECKELIAN AND 
 
 MYLOHYOID GROOVES IN THE JAWS OF 
 
 ME80Z0IC AND REGENT MAMNIALIA 
 
 HV 
 
 H. ART'^I^-R HKXSKKV, R.v 
 
 i.ECTfRKR ox zpowk; :\ tiik rxivi-KsiTV of Toronto 
 
ox TMH IDKNTIl'ICA "<>\ ( (l- MI-CKI'.F.IAS' WD 
 
 MYi.<)HY()n)(;K(»(.\i;s i\ Tin-; j.wvsoi" 
 
 MKSCJZOIC AND Ri;CHN'T MAMMAIJA 
 
 Owen, in his well-known Miiii<.;;r.ii.li of tlu- Miso/nic M.unni.ilia, 
 noted the coinnion occtiruiue nl" ,i line.ir furrow on tln' iiuur ^urf.m' 
 of the jaw in Jurassic niauunals which he doii^Miateil as a "nivloliyi itl 
 groove," and Marsh CS;) has descrihed a similar ;,aoo\e for several 
 species from the Jurassic deposits of Amirica. 'I'lu- pn^^ihk- :,ijriiili- 
 cance of this structure was first commented v m in the dis ission 
 which took place durinj,' the >ears iS^,S-^(^ as to .he mannn u or 
 non-mammalian nature of the ori^^jinal sptiimeus of Iw/i///) //,,// 
 and PhascolothtriiiiH from the Stoueslield .Slate, I)- Ulaiii-. ,.ic ('^S, p. 
 733) havinj; called attention to an inferior m.nijinal <,'r" ne, in Amfilii- 
 thtfiiim, which he rejrardetl as a suture ai ^ ..- iiulicali . A liie com- 
 posite structure of the jaw in ciuestimi. F . iainvilk's opinion was 
 criticized at the time by Dumeril, hut accepted hy (irant ('31^1, the 
 latter considerinjj^ the composite structure of the jaws to he obvious 
 "from the distinct deep fissure extending' alon^r their base between 
 the dental and opercular pieces." Owen ('3.S), on the other hand, 
 believed the jj^rooves to be due to the pressure of a nerve or vessel, 
 although he appears to have been, at the time, a little uncertain of 
 his interpretation, partly, perhaps, on account of the ])resence of a 
 second upper groove in the specimen of P/iiiso>/otfitriuiii, and partly, 
 doubtless, on account of the criticisms of Ogilbv ('■'S) who, while 
 having no explanaiion of his own to offer as to the significance of the 
 groo\xs, objected to their being regarded as of vascular origin. In 
 his " Briti.sh Fos.sil Mammals," published some vears later ('46), how- 
 ever, Owen replied to the opinions of Ogilbv and Craut, and showed 
 that the groove in the specimens of .lii/phi/ii, ■ luw possessed an 
 entire surface, and was therefore not referable ; a suture. In nis 
 subsequent monograph ('71) he repeated his opinion, referring to the 
 groove as " mylohyoid." Following ( )wen, ( )sborn (\S8) has compared 
 the structure with the true mylohyoid groove in the human jaw, at 
 the same time considering it to be of little taxonomic vai .eon account 
 of its variable presence in recent mammals. In the ninth edition of 
 the Encyclopaedia Britauiiica we find the following statement by 
 
 [75] 
 

 ^1 
 
 h'; 
 
 If' 
 
 fill 
 
 4 Bensley : Meckelian and Mylohyoid Grooves 
 
 Flower ('83, p. 376)— "the mylohyoid groove [is] persistent [in 
 Amphitheriuml, as in some of the existing Marsupials and the Whale- 
 bone Whales. This groove, a remnant of that which originally lodges 
 Meckel's cartilage, mistaken for a suture, was once considered evi- 
 dence of the reptilian nature of these jaws." It thus appears that two 
 somewhat similar, but fundamentally distinct structures have been 
 confused under the designation " mylohyoid groove," and it is chiefly 
 towards pointing out their distinction and distribution, and the 
 taxonomic importance of that occurring in the mesozoic Mammalia, 
 that the following remarks' are directed. 
 
 The true mylohyoid groove appears to be typically developed 
 only in the primates and artiodactyl Ungulata, although it is by no 
 means always present in the former, as, for example, in Loris, Lago- 
 thrix, Tarsius, and Galago dcmidoffi. It is fairly frequent in the 
 Edentata {Tatusia, Bradypus, Choltepus). It is present in Lepus 
 among the Rodentia, and possibly also in other forms, but is difficult to 
 identify in this group on accoimt of the presence of other mandibular 
 grooves. As to its general character it is usually a broad superficial 
 furrow (cf. Plate, fig. 3, my.), beginning near and below the dental fora- 
 men and terminating abruptly a short distance forwards. It is fre- 
 quently double, and its extent of development varies somewhat in 
 different individuals. In the human jaw this groove is stated by 
 Quain ('82, p. 56) to lodge the mylohyoid nerve with its accompanying 
 artery and vein. 
 
 The relations of the groove which occurs in the mesozoic Mammalia 
 have been amply illustrated by Owen ('71), Marsh ('87), Osborn ('88), 
 and Goodrich ('94). Two of its modifications are here represented by 
 figs. I and 2 in the plate {Amblotherium, Spalacotherium), copied 
 from Osborn's memoir. Apart from its regular linear outline its more 
 important features are as follows : (a) its close relation with the 
 dental foramen ; in many forms {Amphithcrium, Amblotherium, Am- 
 philestes, Spalacoiherium, Tinodon) it appears to be simply an anterior 
 continuation of the latter ; (b) the fact that while sometimes confined 
 to the posterior part of the jaw {Spalacothen'um, Phascolotheriiim, 
 Aniphilestcs) it frequently traverses the whole length from the dental 
 foramen to the symphysis {Amblotheriumy Achyrodon, Dryolestes, 
 Docodon). As regards the distribution of this groove it is typically 
 
 ' Where not otherwise noted these are based on the osteological collection of Ihe 
 British Museum (Natural History), London. 
 
 [76] 
 
Bensley: Meckelian and Mylohyoid Grooves 5 
 
 developed only in the presumably higher mcsozoic forms, being absent 
 as far as known in the Multitnbercnlata. 
 
 There can be no doubt that Flower's opinion as to the relation 
 of this structure with Meckel's cartilage is the correct one. An 
 exactly similar groove lodging Meckel's cartilage may be seen in 
 embryos of existing mammals, and its somewhat close resemblance to 
 the true mylohvoid groove may be easily shown to be the result of 
 coincidence. In the plate (figs. 10^7-^/) will be foiind illustrations of 
 four transverse sections through the lower jaw of a 6cm. pouch-fcetns 
 of Macropns\ Section a, taken immediately behind the symphysis, 
 passes through what is at this stage the anterior limit of Meckel's 
 cartilage. The symphysial portion of the latter has already been 
 reduced or the anterior part of the jaw has grown beyotul it. This 
 section and section h, which is taken further back, show Meckel's 
 cartilage lodged in a groove on the inner surface of the bony man- 
 dible, and separated by a bony strand from the dental nerve and 
 arter>' in its interior. Section d, taken innuediately behind the dental 
 foramen, shows the dental nerve and artery in close relation with 
 Meckel's cartilage, while section c shows the condition at the foramen 
 where the nerve and arter>- become separated from the cartilage, the 
 two former passing into the body of the jaw while the latter enters 
 the groove on its inner surface. A short distance posterior tf) section 
 d in this series the nerve and artery are seen to give off mylohyoid 
 
 branches. 
 
 A comparison of these sections and of the dissection of a foetal 
 jaw represented in the plate (fig. 5) will suffice to show the identity 
 of the "groove lodging Meckel's cartilage in the embryo with that in 
 the mesozoic Mammalia. The cause of its resemblance to the true 
 mylohyoid groove will also be apparent, since in the einbr\o we find 
 Meckel's cartilage leaving the dental nerve and artery at the dental 
 foramen and passing into a groove on the inner surface of the jaw, in 
 much the same way that, at a later stage, the mylohyoid branches 
 leave the inferior dental trunks at the foramen and pa-^s into the 
 mylohyoid groove. It is probable that the condition described above 
 for Macropus, namely, the lodging of Meckel's cartilage in a groove, 
 represents the general one in the Mammalia. Parker ('85) has de- 
 scribed and figured it for several of the Edentata and Inscctivora, and 
 
 Tpor this specimen, with many others, the writer is indebled to Professor Hash- 
 ford Dean, of Columbia University, New York. 
 
 [77] • 
 
 
 \\ 
 
 I 
 
 :l: 
 
Bensley : Meckelian and Mylohyoid Grooves 
 
 ^ 
 
 I 
 
 1i 
 
 the writer has observed it in the case of several genera of marsupials, 
 including Myrmccobius, Phascogalc, Trichosurus, Phalanger, Dasyu- 
 rus, Perameles, and Thylacomys'. 
 
 It is obvious that in many cases the mylohyoid groove must, 
 during development, become superposed to the Meckelian groove. 
 Magitot and Robin ('62) have described what is apparently that con- 
 dition for man. Rut that such is not always the case may be seen 
 from the forms represented in figs. 8 - and 9 of the plate, in which 
 both grooves are present with similar relations to the dental foramen 
 but with different positions in the jaw. 
 
 Considering^ the nature of the groove represented in the mesozoic 
 Mammalia we can scarcely expect to find it fully developed in adults 
 of recent mammals. Owen ('38, '71) described and figured a groove 
 in the jaw of Myrmecobius, which he regarded as equivalent to that 
 in the mesozoic forms, but Osbom ('88) was unable to recognize this 
 structure in two specimens belonging to the Yale University collec- 
 tion, and he has further stated, on the authority of Mr. Thomas, that 
 it is absent in the British Museum specimens. Leche ('91) also 
 failed to find it in three of his specimens, but has mentioned its pres- 
 ence in a fourth immature one. The fact of the matter is that a 
 short broad furrow does occur in Myrmecobiiis exactly as Owen has 
 described and figured, but its great width almost precludes its being 
 spoken of as a groove, and it has obviously nothing to do either with 
 the mylohyoid or the Meckelian groove. Its presence is due simply 
 to the elevation of the internal alveolar edge. A much more definite 
 groove, due to the same cause, is frequently present in recent mam- 
 mals {cf. Plate, fig. 4). 
 
 Owen also mentioned a similar groove for Phascolomys. This 
 structure, which is amply illustrated in the British Museum speci- 
 mens, appears to represent a mylohyoid groove. In adult jaws it is 
 frequently found to be branched. In a young animal of which the 
 writer dissected this region, the posterior portion of the groove was 
 alone developed, and it lodged the mylohyoid nerve. In the young 
 wombat the groove is placed just at the point where the anterior por- 
 tion of the inflected angle joins the body of the jaw. A similar 
 smicture is frequently present in other marsupials. Its somewhat 
 
 »• J. ^."Jr^ specimens representing these genera were kindly lent~by thelate MrT'Mar 
 tm F. Woodward, of the Royal College of Science, I^ndon. 
 
 A u.' ??"" 'iJ* 'S"" °^ this specimen— a foetal jaw of Piopithecus—ihe: writer is in- 
 debted to Dr. Forsyth Major, of the British Museum, London 
 
 [78] 
 
 
 
 \ 
 
1 
 
 Bensley : Meckelian and Mylohyoid Grooves 7 
 
 different appearance as compared with the mylohyoid groove of plac- 
 ental is due to the presence of the angular inflection. 
 
 Undoubted traces of the Meckelian groove are, however, to be 
 seen in adults of recent mammals, although in most cases only as 
 variations. Fig. 4 of the plate shows the appearance of it in an aged 
 specimen of Dideiphys. This may be compared with fig. 9 which 
 shows the normal condition in a young animal. Figs. 6 and 7 show 
 the condition in two other specimens representing Tatusia and 
 Chrysochloris ■ the former is not fully adult. Similar conditions are 
 observable in some specimens of the following {onmi,— Tnc/iosurits, 
 Phalanger, Perameles, and Petauroides, among the marsupials, 
 Xenurtis and Dasypns among the Edentata, Hcmicentctes and 
 Echinops, among the Insectivora. The groove is doubtless frequently 
 present in many other forms, but such a reduced and variable 
 structure almost defies recognition. As stated by Eschricht and 
 Reinhardt ('66) it is present in the adult of BaliPna mysticctus. 
 
 It is an interesting question why the Meckelian groove is not 
 present in the Multituberculata. Following Cope's suggestion (88) 
 as to the monotreme affinities of these animals, the writer examined 
 the condition in three specimens of Echidna of 2, 9, and i6cm. head 
 and body lengths. The relations in this form, however, proved disap- 
 pointing. In the 2cm. egg-embryo there were only a few traces of 
 bone formation in the lower jaw, while in the two larger animals the 
 dentary element was well formed but was not in relation with Meckel's 
 cartilage posteriorly. In both of the later stages the symphysial 
 portion of the cartilage was seen to be ver>- much elongated, and for 
 a short distance behind the symphysis the cartilage was lodged in a 
 concavity of the dentary bone. Immediately posterior to this point, 
 however', the cartilage was found to leave the jaw, and to pass back- 
 wards independently of it. In the 9cm. embryo its position was in- 
 ternal and ventral with reference to the jaw, and in the older animal 
 its separation from the jaw and its internal position were still more 
 
 marked. , r ^. ^ 
 
 The condition in Echidna is apparently the result of the great 
 reduction or degeneration of the jaw characteristic of this form. It 
 is possible that the conditions in Ornithorhynchus might throw some 
 light on the question, but no embrv-os of this form were available. 
 It seems most unlikely that in the Multituberculata the Meckelian 
 cartilage could have had the same relations as in existing mammals, 
 
 [79] 
 
 ? I ■ \ 
 
 
 
8 Bensley: Meckelian and Mylohyoid Grooves 
 
 and was abs. it in the adult stage. There is a possibility that in the 
 mammalian prototypf <he cartilage was either completely enclosed in 
 the dentary bone or eoKJssified with it. It is interesting to note that 
 Parker {op. cit.) has described a partial ossification of the cartilage in 
 the yoimg of Centetcs and Talpa, and a partial enclosure of it in the 
 latter form and in Erinacetis. The frequent exposure of Meckel's 
 cartilage in the jaws of lower Vertebrata, however, warns against the 
 adoption of such an explanation before the acquisition of more definite 
 evidence. 
 
 i8SS 
 •66 
 •94 
 
 American Naturalist, March, 
 
 '9> 
 
 BIBLIOGRAPHY 
 
 Cope, E. D., The Multituberculata Monotreraes 
 1 888, p. 259. 
 
 11^::'^^^^.:,^:^:;;%^,"^^^ ^"^ '^ p'-^^-'" ^■''^■p'^e desto„esfieid. 
 
 Eschricht, D. F., and Reinhardt, I., On the Greenland Ritrlit w»,<.i-. /n 1 
 mysticetns, Linn.) (Translation)."' Ray Soc. , i ^ pp i!^,'f '' ^^"^^ ^'^^'^"^ 
 Flower, W. H.. Article " Mammalia." Encyclopcedia Briunnica, 9th Ed., 
 
 j^^v.ici^-v<^"x::^ir^i"«^nii^ir ^•''^^'--^^"^ «>-• Q-t- 
 
 Grant, R. E., Thompson's British Annual. 
 
 •62 
 
 •S7 
 '38 
 
 •88 
 
 ■3» 
 
 ■46 
 '71 
 
 '85 
 '82 
 
 Quoted in full by Owen (-46, p. 38) 
 
 Leche, W. Beitrage zur Anatomie des Myrmecobius fasciatus Verb d Rio 
 Vereins in Stockholm, Bd. iii., Xr. 8, S. 136-154. •«»»-""•"». vern. a. Hiol. 
 
 Magitot, E., and Robin Ch. M.-.„„ire surun organe transitoire de la vie frPtal,. 
 
 ^r^:^. '!.TZ ^''■■'•'''*'"'''^ "-''^'- Ann^Sci.Nat., iv met' ricTo^Sei 
 
 seriet'p?." 3'':>'7-3''48:""'" ^"""' '''"""'"^- -'■"• J°"^"- «"- -'• «-»- 3rd 
 Ogilby W., Observations on the Structure and Relations of the presumed M»r 
 
 ^iSvv^^z '' ''°"''- ^''^- ^^°'- soc LoS^rvoLiri":: 
 
 Osborn, H. F.. The Structure and Clns«ifi><!itiV,ti „f *%,„ »» 
 
 Jour. Acad. Nat. Sci. Philadelphia vol ix No 2, pp. ^Is-S"'"'' ^'"""""''''- 
 
 Owen, R., Observations on the Fossik renrpui.MiiM.r n.« Ti 1 ., 
 
 ii (Valenciennes), etc. Trans. <^:^t^:^Z\T.^ ^'ll^^^X^'^Z' l^^^^^^^ 
 
 Owen, R., British Fossil Mammals. London 1846 4^, pp. 47 05. 
 
 ?arontograpSsT', tn^;„'"^'' ^^^'""'="'^ °^ "^ ^'--- ^°-atio„s. 
 Parker, W. K.. On the Structure and Develooment of the <5lr •• ;„ n. ii» 
 176, p^r "75.^''^"""' ' '^" "'•• ^"-''voraP."'Tr„l't?:- .!" ^'o^nd^n^^S: 
 Quain's Anatomy (Thomson, Schafer, and Thane), 9th ed vol i 
 
 [So] 
 
 BilH 
 
Bensley: Meckeuan and Mylohyoid GRf^ovES 
 
 Fig. I. 
 
 " 2. 
 
 " 3- 
 
 " 4- 
 
 " 5- 
 
 " 6. 
 
 " 7. 
 
 " 8. 
 
 " 9. 
 
 " lo, 
 
 EXPLANATION OF PLATE 
 
 Amblothenum soricinnm. Right mandibular ramus, showing the groove for 
 Meckel's cartilage. (After ^ U>Tn. 
 
 Spalacothenum tricuspidf Left ramus reversed, si.. "K tljf'^')"^^'"" °; 
 the groove for Meckel's cnilage with the dental forami (After Osbom.) 
 Mycetes ursinus. Right ramus, showing the true mylohyoid groove. 
 Didelphyii marsupialis. Right ramus of an old individual, in which traces of 
 the groove tor Meckel's cartilage are present as a variation. 
 Macropus sp. Moist preparation of the l.'t ramus of a ycm. pouch-foetus, 
 showing Meckol" cartilage lodged in its groove. The car-ljonea are schemati- 
 cally represented 
 
 Tatusia novemcincta. Right ramus of an immature individual, showing the 
 groove for Meckel's cartilage and the mylohyoid groove below it. 
 Chryiochoris trevelyanus. Left ramus, showing traces of the groove for 
 Meckel's cartilage in the adult. 
 
 Propithecui tp. Right ramus of foetus ; the mylohyoid groove is here formed 
 below that lodging Meckel's cartilage. 
 
 Didefphys mc -supialis. Right ramus of an young individual showing the nor- 
 mal .ippearance of the groove for Meckel's cartilage in the later stages of its 
 reduction. 
 
 a-d Macropus sp. Transverse sections through th; right ramus of a 6cm 
 pouch-fcetus, showing the relations of Meckel's cartilage and tne dental nerve 
 and artery to the jaw. For explanation see text. 
 
 Abbreviations 
 
 u,g_groove Tor Meckel's cartilage. 
 
 my— mylohyoid groove. 
 
 mc — Meckel's cartilage. 
 
 ml — malleus. 
 
 i — incus. 
 
 st — stapes. 
 
 ty— tympanic annulus. 
 
 n— dental nerve. 
 
 a — dental artery, 
 
 c— coronoid process of mandible. 
 
 fm— masseteric foramen. 
 
 '■% i 
 
 
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 [8i] 
 
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 I 
 
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