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Show 72 ZOOLOGY OF TilE VOYAGE OF TilE DEAGLE. . . f the lower jaw is peculiarly re- In the Megatberium the mfenor contour 0 · · e or enlarO'ement . h b d for the convex prommenc ' o markable, as Cuvter as o serve ' I tl c Mylodon the corre- . . 1 d ·d f· m its middle part. n 1 wluch IS developec ownwai s 10 d' g that which may . . . sr ght deO'ree not excee tn sponding convex tty extsts m ~ very 1 0 ' •• aw of the Ai, or Orycterope. be observed at the correspondmg part of the lower .J . f h . lose to the A broad and shallow furrow extends along the outer Side 0 t e P w' c . d t alveolar margin, from the beginnm. g o f' t h e coronot·( 1 process to the antenor en a 1 foramen. . . ' t . 1 and posterior to the last The base of the coronOid process begms ex em a . d grinder: the whole of the ascending ramus of the jaw, ben~ath thedcolrbonlm rocess is excavated on t.t s m. ner st'd e b y aWl' d e an d deep concavtty ' boun ec. e ow Ey a well-marked ridge, which extends obliquely backwards from the po~tenor part of the alveolus of the last grinder to the inferior margin of the ascendmg ramus, which is bent inwards before it reaches the angle of the jaw. . The large foramen or entry to the dental c~nal is situ~ted in the mtern~l c~~- cavity of the ascending ramus of the jaw, two mches behmd the last molat, tluee inches from the lower margin of the· ramus, and nearly five m· e 11 es ft·o m th. e elevated angle of the jaw: it measures nine lines in the vertical. diameter, and Its magnitude indicates the large size of the vessels which are destmed to supply ~he materials for the constant renewal of the dental substance,- a substance wlnch from its texture must be supposed to have been subject to rapid abrasion .. Abo~1t an inch behind the dental foramen a deep vascular groove, about two lines m breadth, is continued downwards to theridgewhich circumscribes the internal concavity of this part of the jaw, and perforates the ridge, which thus arche.s o~er tl~e canal: this structure is present in both rami of the jaw. The mylo-hymd ndge 18 distinctly marked about an inch and a half below the alveolar margin. Other muscular ridges and irregular eminences are present on the outer side of the base of the ascending ramus, and neat· the angle of the jaw; as shown in fig. 1, Pl. XIX. From the preceding descriptions it will be seen that the lower jaw of the Mylodon is very different from that of the lJfegatherittrn; with that of the Megalonyx we have at present no means of comparing it. Among existing Edentata the Mylodon, in the form of the posterior part and angle of the jaw, holds an intermediate place between the Ai and the great Armadillo; in the form of the anchylosed symphysis of the lower jaw it resembles most closely the Unau or two-toed Sloth; but in the peculiar external configuration of the Rymphysis resulting from the mammilloid processes above described, the Mylodon presents a character which has not hitherto been observed in any other species of Bntla, either recent or fossil. In conclusion it may be stated, that the teeth and bones here described offer FOSSIL MAMMALIA. 73 all the conditions and appearances of those of a full grown animal ; and that they present a marked difference of size as compared with those of the Mylodon, I-larlani, as will be evident by the following admeasurements. ADllrEASUREMENTS OF TUE LOWER JAW OF 1\IYLODON DARWINII, Length (as far as complete) Extreme width, from the outside of one ramus to that of the other Depth of each ramus Length of alveolar series . From nrst molar to broken end of symphysis Breadth of symphysis Longitudinal extent of symphysis Circumference of narrowest part of each ramus Inches. Lines. 17 DESCRIPTION OF A CONSIDERABLE PART OF THE SKELETON OF A LARGE EDENTATE MAMMAL, ALLIED TO THE MEGATHERIUM AND ORYCTEROPUS, AND FOR WHICH IS PROPOSED THE NAME OF SCELIDOTHERIUM* LEPTOCEPHALUM. OF the large Edentate quadrupeds that once existed in the New World, sufficient of the osseous remains of the gigantic Megatherium alone has been transmitted to Europe to give a satisfactory idea of the general fo1·m and proportions of the extinct animal. Different bones of the Megalonyx, Mylodon, and Glyptodon have been described, but not sufficient of the remains of any individual of these subgenera has, hitherto, reached Europe, or been so described as to enable us to form a comparison between them and the Megatherium, or any of the existing Edentata, in regard to the general construction and proportions of the entire skeleton. This state of our knowledge of the osteology of the singular giants of the Edentate Order renders the remains of the present animal peculiarly interesting, since, although the extremities are too imperfect to enable us to reconstruct the entire skeleton, a sufficient proportion of it has been preserved in the natural position to give a very satisfactory idea of its affinities to other Edentata, whose osteology is more completely known. * l:"cXts,femur; 8qpwv, bellua; in allusion to the disproportionate size of tho thigh-bone. L |