152 



In the existing Sloths the sole of the hind-foot is, indeed, turned inwards, 

 as it is in the Orangs and other excellent climbers ; but the construction of 

 the foot itself, not its mere inversion, determines its scansorial function. The 

 Quadrumana, for example, enjoy this faculty by virtue of the hinder opposable 

 thumb ; and the Sloths by a so much greater degree of inversion of the foot than 

 in the Megatherioids, and by so different a structure of the articulation of the 

 tarsus to the leg, that the power of supporting and transporting the body on 

 plane ground is sacrificed to the acquisition of the prehensile faculty. 



The absence of the pivoted articulation of the astragalus to the fibula, and 

 the introduction of the new modification of the tibial part of the ankle-joint, 

 whereby a process of the astragalus fills a cavity in the part of the tibia from 

 which, in the Sloths, the inner malleolus projects, are alone sufficient to suggest 

 a corresponding difference in the actions to which the hind-feet of the gigantic 

 extinct phyllophagous Edentata were appUed. 



The scansorial hypothesis of the Megatherioids is directly opposed by almost 

 every other characteristic of their known organization, except the very few which 

 might at first view suggest it. Thus, if the Megatherioids had been limited to 

 the act of climbing in order to obtain their food, they could only have browsed 

 on the largest and strongest brandies of even such gigantic trees as Dr. Lund has 

 conjectured to have coexisted with them : to animals of their size and weight the 

 greatest proportion of the tree, supporting the most succulent, nutritive and 

 abundant sprouts and leaves, which the fight and slender Sloths now easily attain, 

 must have been inaccessible, and have waved or shook before their hungry 

 gaze in tantalizing unreachableness. By the modifications of the Bradypodal 

 structure which gave the Megatherioids power to uprend the whole tree, every 

 portion of their peculiar and coveted food would be brought within their reach ; 

 once prostrated, it could be disbranched and stripped of its foliage at leisure. 



The inversion of the sole of the foot, slight as it is in the Mylodon and smaller 

 Megatherioids, is attended with secondary advantages in addition to that chief 

 one of securing to them the full use of both claws and hoofs ; thus as the delver 

 penetrates the soil more easily when the spade is driven in a sloping direction, 

 so the hind claws, when employed in digging, would derive a like advantage 

 from their oblique position. Also in the veiy possible case of trees too large to 

 be uprooted by, yet with foliage tempting to, the smaller Megatherioids, they 



