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Show ZOOLOGY OF TliE VOYAGE OF TilE BEAGLE. b . 1 ut resembles that of 68 I Tl Of tlle tooth presents no protu erances, l tent IS. 1e crown the Sloth· the roots are hollow.""* b ding Thi 'ros il i r ferred by Dr. Harlan to his iJ'/egalonyx l~queatus; ut, pen b . . f · es in wlnch as may be seen Y the ab ence of other proof of the H1entlty o spec! , l d'ff' .' 'dely in form it . · PI XVII the teet 1 1 e1 w1 ' comp::uino· fig 2 with fia. 4, m · ., . 1 1 · 1 ' b : ' d t ch an approximation on 1ypot 1et1ca · would be obv1ously hazardou to a op su < ' • • around .t In order, howeYer, to obtain more satisfaetory evidence of the nat~ne ;nd amount of the uifD ren e between the llfegalouyx laquea~us, and .the allied animalrcprc ented by the above-described fragment .of lower JaW, I wiot_e to m~ mu h re p ct d friend l\1. LAURILLARU, reque~ting hun to send ~e a sketch 0 the t eth in the cast of that lower jaw, wh1ch had been transmtt~e~ from ~.ew York to the Garden of Plants. 'Vith full confidence in the.c~w.ractenstlc precisiOn and accuracy of the drawing with which I have b~en obhgmgly ~avoure~ by M. Laurillard, I am dispo ed to regard the amount of <.hfference rec?gmzable m every tooth in the lower J·a W m· questw· n (fi g, 3 an d 4,) as compared wtth the molar tooth either of 1l[eo·alonyx J qj'ersonii (fig. 1 ,) or JUeg"'. laqueatus (fig. 2) to be such as to ju tify it generic separation from .11Iegalon!JX on the same grounds as J.lf~g~louyx i distinguished from 11Ieo·atheriwn, and for the subgenus of Megathe.noul. Edcnla.ta, thus indicated, I would propo e the name of 1\iYLOUON.t '!'he species of which the fossil remains ar described by Dr. Harlan may be ded1cated to that iudefati o-able Naturalist who has contributed to natural science so much valuable information re pecting the Zoology, both recent and fossil, of theN orth American continent. The fossil about to be described represent a second and smaller pecie of th arne genus, and I propo e to call it ]}fylodon Darwinii, in honour of it discoverer, of whose 1·es arches in the Southern division of the New \Vorld it form one of man) new and interesting fruits. Harlan's ~Iodical and Phy ical Researches, I 835, p. 33.J.. llf. de Blaiuville speaks of a ca t of a fra o-ment of a lower jaw ' portant encore cinq dents en seric ;" as having been tran mittcd to the Museum of the Garden of Plant· from North America, tog thcr 'vith other bones, all of which ho r<' for to the genus ltfeualonyx; M. de Blainville docs not de cribe these teeth, which is to be regretted, ina much as, if he b<' correct in regard to their number which can hardly be doubted, and if he wrote with any clear and definite ideas of the generic characters of Jfcgalonyx, this would indicate that lJJ<'galonyx differed g0ncrically both from lJ!egatlterium and .llylodun in a more important dental character than has hitherto been uspcctcd ( co " omptcs Rend us, &c." 1 30 Xo. ,-. p. 142.) t Dr. Harlan also indicates difference in cert.."'in part of the keleton of the r cw York fo ils as compared with his .:lft'[J.r. laqucatus ; but thinks them probably due to a ditl'ercnce in the age of the individuals : he ays ' There is also in l\Ir. Graves' collection, inN w York, a tibia, nearly perfect from the right leg; the cgment of a fiatteucd pherc ou which the external condyle of the femur mo'ie , i rather more dcpres cd, than in the spccitu<'ll from Dig-bone-cave. Other mark~ and peculiarities arc ob ervablc on this bone, not found on that of the •'~~'!l"lonyx laqucatus of Big-bone-cave, but t!Jcy arc probably due to a difference in the age of the indiYidual ·." Loc. cit. p. 335. t Ill uAij, 111ola; ocouc, dens. FOSSIL 1\IAMMALIA. (39 'l'his fossil was discovered in a bed of partly consolidated gravel at the base of the cliff called Punta Alta, at Bahia Blanca in Northern Patagonia: it consists of the lower jaw with the series of teeth entire on both sides: but the extremity of the symphysis, the coronoid and condyloid processes, and the angular process of the left ramus, are wanting. 'rhe teeth are composed, as in Bmdypus, Megatlterium and lJiegalonyx, of a central pillar of coarse ivory, immediately invested with a thin layer of fine anrl dense ivory, and the whole surrounded by a thick coating of cement. In the fig. 5, Pl. XVII., the fine ivory is represented by the white striated concentric tract on the grinding surface of the teeth ; it is of a yellowish-white colour in the fossil, and stands out, as an obtuse ridge, from that surface: both these conditions depend on the large proportion of the mineral to the animal constituent in this substance of the tooth. The external layer of the cement presents in the fossil the same yellowish-brown tint as the bone itself, which it so closely resembles, both in intimate structure and in chemical composition; the internal layer next the dense ivory i jet black, indicating the great proportion of animal matter originally present in this part. The central pillar of coarse ivory, which, from its more yielding texture, has been worn down into a hollow at the triturating surface of the tooth, also presents, as a consequence of the less proportion of the hardening phosphates, a uarker brown colour than the external layer of the cement, or the bune itself. The teeth are implanted in very deep sockets; about one-sixth only of the last molar projects above the alvcol us ; the proportion of the exposed part of the tooth increases as they are placed further forwards. The implanted part of each tooth is simple ; preserving the same size and form as the projecting cl'Own, and presenting a large conical cavity at the base, indicative of the original persistent pulp, and perpetual growth of these teeth. The extent of the whole four alveoli is four inches, eight lines; the length of the jaw from the angle to the broken end of the symphysis is seventeen inches and a half;'* from the figures it will be seen that only a small proportion of the anterior part of the jaw is lost, so that we may regard the dentigerous part of the jaw as being limited to about one-fourth of its entire length; the alveoli being nearly equidistant from the two extremities. The fil'st and second teeth, counting backwards, are separated by an interspace of rather more than three lines; that between the second and third is one line less ; the third and fourth are rather more than a line apart: from the oblique position, however, of the three hinder teeth the intervals between them appear in a side view, as in fig. 1, Pl. XIX., to be less than in reality, and the third and fomth teeth seem to touch each other. ·ll- If the lower jaw of 11-fylodon Jlarlani, boars tho same proportion to its teeth as does that of Jlfylodo11, Dar1cinit, it must bo about two feet ir1 length. |