87 



in the Megatherium and Megalonyx. The anterior part of the distal end of the 

 radius is more concave in the Mylodon than in the Megatherium. The distal 

 articular extremity has its posterior boundary shorter transversely, and more 

 produced downwards than in the Megatherium. In the Scelidotherium the ge- 

 neral proportions of the radius much more nearly approach those of the Mylodon, 

 but the proximal articular cavity is subcircular, as in the Megatherium and Me- 

 galonyx. The distal half of the outer margin of the radius is convex : the bici- 

 pital tuberosity is nearer the proximal end, and nearer the inner margin of the 

 bone : the posterior surface of the radius more resembles that in the Megalonyx. 

 Like the perforated humerus, the present bone in the Scelidothere also exhibits 

 modifications which connect the Mylodon with the Megalonyx. 



Both the radius and ulna of the Mylodon are destitute of a medullary cavity, 

 Uke the same bones in the Sloths. The radius of the Mylodon presents, however, 

 a small orifice for the medullary artery on the outer side, about two inches from 

 the proximal end, the canal of which leads obliquely downwards to an irregular 

 cavity in the middle of the bone, three lines in length by one line in breadth : 

 this rudiment of a medullary cavity communicates by many apertures with the 

 surrounding closer cancellous texture. 



Bones of the Manus or Fore-foot *. — The fore-foot of the Mylodon participates 

 with the fore-arm and arm in the characteristic massiveness and strength of its 

 bony fabric. It is pentadactyle, rather short in proportion to its breadth, and 

 has evidently been provided with both claws and hoofs, or had a callous cover- 

 ing for the stunted terminations of two of the digits, analogous to a hoof. In 

 the reahty of this, hitherto unknown, combination of ungulate and unguiculate 

 characters in the same animal, full confidence may be placed, for the series of 

 bones of both fore-feet of the Mylodon have been collected and transmitted in so 

 complete a state as to leave nothing to be guessed at or desired in the acquisi- 

 tion of just ideas of this complicated and significant part of its Osteology. 



Carpus. — The carpus includes seven bones, four in the first, and three in the 

 second row, or rather there are three bones proper to each row, and one which 

 is common to both. This latter bone is the scaphoides (a), which, besides its 

 usual relations to the radius, lunare, and trapezoides, sends down a process (a') 



* Plates XV. and XVI. 



