FOSSIL MAMMALIA. 93 



and higher organized climbers, in like manner, depend mainly on their posterior 

 claspers in descending trees, and hold on by means of them whilst selecting the 

 place for the next application 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 advan- 

 tageous structure of the hand in the Quadrumanous species, and notwithstanding 

 that the tail is for this purpose sometimes specially organized to serve as a pre- 

 hensile instrument, 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 

 the active exertions of climbing, so they are the strongest as well as the longest 

 in all the best climbers, and the weight of the body which they have to drag along 

 is diminished by dwarfish proportions of the hinder 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 ex- 

 ceed 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 PI. 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 mar- 

 row 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 



