148 



by the action of the muscles towards the grasped branch, as to a fixed point. 

 The stouter proportions of the prehensile hooks of the Mylodon accord with 

 the harder task of overcoming the resistance of the part seized and bring- 

 ing it down to the body. For the long and slender brachial and anti-brachial 

 bones of the climbing Sloth we find substituted in its gigantic predecessor a 

 humerus, radius and ulna of more robust proportions, — of such proportions, 

 indeed, in the Mylodon robustus, as are unequalled in any other known existing 

 or extinct animal. The tree being thus partly undermined and firmly grappled 

 with, the muscles of the trunk, the pelvis and hind limbs, animated by the 

 nervous influence of the unusually large spinal cord, would combine their forces 

 with those of the anterior members in the efforts at prostration. And now let 

 us picture to ourselves the massive frame of the Megatherium, convulsed with 

 the mighty wrestling, every vibrating fibre reacting upon its bony attachment 

 with a force which the sharp and strong crests and apophyses loudly bespeak : 

 — extraordinary must have been the strength and proportions of that tree, 

 which rocked to and fro, to right and left, in such an embrace, could long with- 

 stand the efforts of its ponderous assailant. 



A few observations remain to be offered touching the most singular modifica- 

 tions of the feet of the Mylodon, viz. the thick and stunted outer toes, which 

 were evidently enveloped in a kind of hoof It would be difficult to conceive 

 any modification of the Sloth's structure, whereby such a creature might, with 

 dimensions rivalling those of the largest Pachyderms and unfitting it for an 

 abode in trees, still continue to derive from them its sustenance, more simple 

 and more effectual than those which the skeletons of the Mylodon and Mega- 

 therium have brought to hght. Their power of standing and walking freely on 

 the ground was gained by the addition of these strong hoofed digits to a tridac- 

 tyle or didactyle unguiculate and prehensile foot. With this addition some 

 minor modifications in the proportions of the claws were combined, and strength 

 was given to other parts of the frame, to enable the animal to prostrate the trees 

 it could not climb. 



How admirably the superadded parts of the feet, employed in sustaining the 

 Megatherioids in their heavy march, have been introduced, a glance at the co- 

 adapted bones will show. In the fore-foot, for example*, which, in the ordinary 



* See Plate XV. 



