86 



Megalonyx has the same distal convexity for articulating with the radius, and de- 

 viates still further than in the Megatherium from the Mylodon in the length and 

 slenderness of the shaft of the bone, and in the feeble development of the mus- 

 cular ridges. The general proportions of the idna of the Scelidotherium are in- 

 termediate between those of the Megatherium and Mylodon. 



Radius *. — As the entire radius of the Megatherium and Scelidotherium, and 

 the cast of that of the Megalonyx, form part of the collection of Fossils in the 

 Museum of the College, I shall combine, with the description of the radius of the 

 Mylodon, that of the same bone in its extinct congeners. 



The radius of the Mylodon is thicker in proportion to its length, stronger, and 

 much more deeply impressed by the muscles of the fore-arm, more especially by 

 the extensors of the hand, than in the Megatherium or Megalonyx ; it differs in 

 the shape of the proximal articular cavity, which is more oblong ; the marginal 

 surface which rotates upon the ulna is narrower and more convex. The rough 

 tuberosity for the insertion of the biceps is further from the proximal joint and 

 more advantageous for the action of the muscle in the Mylodon. The rough ex- 

 ternal margin of the radius, which, in the Megatherium, partially subsides and 

 becomes thinner as it extends downwards, and which would appear to stand out 

 as adistinctprocessin the Madrid specimen, gradually increases in the Mylodon 

 until it expands into the tuberosity above the styloid process. The most marked 

 modifications of the Mylodon's radius are seen by comparing its posterior surface 

 with that in the Megatherium. In the more gigantic Edental this surface pre- 

 sents a moderate and even convexity, whilst in the Mylodon f it is traversed by 

 a longitudinal ridge or elevation, separating an outer from an inner depression. 

 Of these depressions the outer one is the broadest and deepest ; and between it 

 and the radial edge there are two narrower longitudinal canals, bounded by well- 

 defined rough ridges. The internal longitudinal shallower cavity is continued 

 obliquely across the distal end of the bone, impressing that part, above the sty- 

 loid process, with a well-marked canal, which is wanting in the Megatherium ; 

 the angular eminence bounding the distal side of that depression is represented, 

 in the Megatherium, by a convex rough protuberance. The longitudinal con- 

 tour of the posterior part of the radius is concave in the Mylodon, but convex 

 * Plate XIV. figs. 3, 6, 7, 8, 9. t Hate XIV. fig. 6. 



