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Show 92 ZOOLOGY OF TilE VOYAGE OF TilE BEAGLE. h ffi 't f the Scelidothere with the ness : but in all these circumstances t e a m y o. d . d 1 . · There 1s no other known qua I upe Megathere is prominently hroug 1t mto VIew. d' d · h' t In procee mg with which the Scelidothere so closely correspon s 111 t IS respec ·. d. d ' however to compare together the thigh-bones of these two ext~nct q~a lwpet:· several differences present themselves, which are worthy of .notice: 0 t lese , e first is the presence in the Scelidothere of a depressi~n for.a 'ligamentum teres 0 ~ the back part of the head of the femur, near its junctwn With the neck of the b~ne · this is shewn in the posten·o r vi·e w of t 11 e 11.' emur g1• ven 1• 11 Pl XX The head Itself • • • • forms a pretty regular hemisphere: the great trochanter does not nse so high as in the Megatherium, but, relatively, it emulates it in breadth: the small trochanter is proportionally more developed: the external contour of the shaft o.f the .femur is straighter in the Scelidothere than in the Megathere, and the shaf~ Itself IS less bowed forwards at that part. The articular condyles occupy a relatively sm~ller space upon the distal extremity of the femur in the Scelidot~ere, and th~y differ more strikingly from those of the Megathere, in being contmued one mto tl~e other: the rotular surface, for example, which is shewn in fig. 5. Pl. XXV· lS formed by both condyles, while in the Megatherium it is a continuation exclusively of the external articular surface. The patella, which works upon the above-mentioned surface, is a thick strong ovate bone, with the smaller end downwards : rough and convex externally, smooth on the internal surface, which is concave in the vertical and convex in the transverse directions. Of the bones of the leg only the proximal end of the tibia is preserved; but this is valuable, as shewing another well-marked difference between the Scelidothere and Megathere ; for whereas in the latter the fibula is anchylosed with the tibia, this bone, in the Scelidothere, presents a smooth flat oval articular surface, which is shewn in fig. 2. Pl. XXVII. below the outer part of the head of the bone ; from the size and appearance of which, I infer, that the fibula would not have become confluent with the tibia, even in the mature and full-grown animal. The relative length of the fore and hind extremities cannot be precisely determined from the present imperfect skeleton of the Scelidothere; but there is good evidence for believing, that the fore extremity was the shortest. The humerus is shorter than the femur by one-ninth part of the latter bone; and the radius, which wants only the distal epiphysis, must have been shorter than the humerus . Now the relative development of the fore and hind legs is one of the points to be taken into consideration in an attempt to determine the habits and nature of an extinct mammal. In climbing animals the prehensile power is more essential to the hinder than to the fore parts or extremities. In the leech the principal sucker is in the tail ; FOSSIL MAMMALIA. 93 and high~r organiz~d climbers, in like manner, depend mainly on their posterior claspers m descendmg .tre~s, and hold on by means of them whilst selecting the place for the next apphcatwn of those at the fore part of the body, whether their place be supplied by the beak, as in the Maccaws, or the fore-feet or hands in the Mammalia. But, although we perceive the hinder limbs to be the last to lose the advantageous st~u~ture of ~he hand in the Quadrumanous species, and notwithstanding that ~he. tail Is for tlus purpose sometimes specially organized to serve as a prehensile mstrument, yet we find that the power of grasping the branches of trees by either legs or tail is never maintained at the expense of undue bulk and weight of those organs. On the contrary, as the fore-limbs are the main instruments in ~he active exert~ons of climbing, so they are the strongest as well as the longest ~n a.ll t.h~ best climbers: and the w~ight of the b~dy w~ich they have to drag along IS dimimshed by dwarfish proportiOns of the hmder limbs, as in the Orangs and the Sloths. Can those huge quadrupeds have been destined to climb that had the pelvis and hinder extremities more ponderous and bulky in proportion to the fore-parts of the body than in any other known existing or extinct vertebrate animals? M. Lund argues for the scansorial character of the Megalonyx, because its anterior extremities are longer than the posterior ones; but if they somewhat exceed the hind-legs in length, how vastly inferior are they in respect of their breadth and thickness. The prehensile faculty of the hinder limbs of the best climbers, as the Sloths, Orangs, and Chameleons is by no means dependent on the superior mass of muscle and bone which enters into their conformation, but is associated with the very reverse conditions. It is impossible to survey the discrepancy of size between the femur and the humerus of the Scelidothere, as exhibited in Pl. XX., without a conviction that it relates to other habits than those of climbing trees. The expanse of the sacrum, the evidence of the muscular masses employed in working the hind legs and tail, which is afforded by the capacity of the cavity lodging the part of the spinal marrow from which the nerves of those muscles were derived, both indicate the actions of the hind-legs and tail to have been more powerful and energetic than would be required for mere prehension: and the association of hinder extremities so remarkable for their bulk, with a long and powerful tail, forbids my yielding assent to the speculation set forth by M. Lund, as to the prehensile character of the tail of the Megalonyx. Astragalus.-In the examination of this characteristic bone I have kept in view the question of the habits of the Megatherioid quadrupeds in general, and the especial affinities of the Scelidotherium, in illustration of which I shall notice at |