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L,l.r:ir\ ( (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 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>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 S ! :i^ [8i] . ? i ■ I i ■ ! i u * it K ,'i lo'- V: