82 



therium, the degree of convexity is greater, the head rises higher, and is sepa- 

 rated by deeper depressions from the outer and inner tuberosities : again, the 

 inner tuberosity is the largest in the Megalonyx, but the smallest in the Mylodon. 

 The deltoidal tract, especially the ridge or margin which overhangs the musculo- 

 spiral canal, is comparatively very feebly developed in the Megalonyx, and is 

 even less strongly marked in the Megatherium than in the Mylodon. The 

 musculo-spiral canal is of great breadth and depth, and divides the deltoid ridge 

 from the strong process which is continued outwards from the back and outer 

 part of the distal third of the humerus to form the supinator plate above the ex- 

 ternal condyle *. 



The vertical outline of the back part of the Mylodon's humerus is slightly 

 concave ; in the Megalonyx it presents a double curve, being slightly convex at 

 its lower part. The posterior surface of the broad distal portion of the humerus 

 in the Mylodon is slightly concave from side to side, and the depression for the 

 olecranon is feebly marked at its lower part : this is much deeper in the Mega- 

 lonyx. The internal supra-condyloid, or pronator plate, has a pointed form in 

 the Megalonyx and Megatherium, but in the Mylodon it has a greater relative 

 vertical extent, with a convex outline. The thicker and shorter shaft of the 

 Mylodon's humerus makes the expansion of the distal end, great as it is, less 

 sudden and disproportionate than in the hammer-shaped humerus of the Mega- 

 therium. 



In one humerus there was no trace of a medullary artery ; in another the orir 

 fice was present, but of unusually small size, a httle below the middle of the 

 posterior surface of the bone. Tlie canal led directly, widening a little, to a 

 small cell or rudiment of a medullary cavity in the centre of the cancellous sub- 

 stance ; the arterial canal there divided into two or three branches ; the principal 

 of which passed straight to the ulnar surface, through the cancellous and dense 

 outer wall, to open by a small foramen two inches above the internal condyloid 

 plate. 



On comparing the humerus of the Mylodon with that of existing Mammalia, 

 we find its characters more closely repeated by the fossorial Ant-eaters and Arma- 

 dillos, than by the arboreal Sloths, yet not so closely as by the gigantic extinct 



* Plate XII. 



