107 



the casts of the series of bones on which the genus Megalonyx was originally 

 founded, afford the means of a comparison with their analogues in the Mylodon, 

 which will be followed in the order in which the bones of the Megalonyx have 

 been described by Cuvier. 



The ungual phalanx of the middle digit of the left fore-foot* is of the same 

 length and depth as in the Mylodon, but has only half the breadth ; the median 

 ridge of the articular surface is sharper ; the position of the joint which favours 

 the downward inflection of the claw, and that of the inferior perforated osseous 

 plate, are the same in both species. The osseous sheath is more developed in 

 the Mylodon, and the upper margin of the claw, which in the Megalonyx is 

 trenchant, is in the Mylodon broadly convex, with a longitudinal indentation 

 near the apex. 



The second phalanx f of the middle digit of the Megalonyx is twice as long in 

 proportion to its breadth as that of the Mylodon, and has a more symmetrical 

 figure. The distal trochlea is narrower, but with a much deeper median canal. 

 The median ridge of the proximal articulation is more developed, and the infe- 

 rior boundary of that articulation is more produced than in the Mylodon. 



As the second and ungual phalanges of the second digit in the Megalonyx are 

 equal in size to those of the third, they differ in a corresponding degree from 

 their analogues in the Mylodon, which are much less than those of the third digit. 



The proximal phalanx | of the third or middle digit in the Megalonyx corre- 

 sponds more closely than the other phalanges with that in the Mylodon ; but its 

 characteristic abbreviation is the more remarkable in the Megalonyx on account 

 of the greater length of the second phalanx. A greater proportional vertical 

 diameter of the bone, and a greater depth of the proximal and distal articular 

 canals, especially of the former, are the only modifications that distinguish this 

 proximal phalanx in the Megalonyx. 



The general proportions of the middle metacarpal bone of the Megalonyx ^ 

 are very similar to those in the Mylodon ; but in the more vertical position and 

 greater projection of the distal articular ridge, and in the flatter under surface 

 of the body of the bone, which is more distinctly separated from the radial sur- 

 face, the Megalonyx more closely resembles the Megatherium. The ulnar 

 angle of the base of this metacarpal is proportionally less produced in the Me- 



* Ossem. Fossiles, 1836,8vo, pi. 216,fig. 1. t Ibid. fig. 2. : Ibid. fig. 3. § Ibid. fig. 4. 



o 2 



