72 



portion is produced forwards upon the anterior cmgle of the ulna, and 

 is very slightly concave ; the outer division is more deeply excavated. 

 The articular surface is continued for a few lines upon the large rough 

 depression for the head of the radius. Purchased. 



418. The proximal part of the right radius of the Mylodon robustus. 



Purchased. 



419. The distal extremity of the same radius. 



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 Megathe- 

 rium 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 ad- 

 vantageously situated for the action of the muscles in the Mylodon. 

 The rough external 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 a distinct process in the Madrid spe- 

 cimen, gradually increases in the Mylodon until it expands into the 

 tuberosity above the styloid process. Purchased. 



420. A fragment of the distal extremity of the left radius of probably the 



same individual : it includes the styloid process which is adapted to the 

 concavity of the articular surface of the scapho-trapezial bone. 



The anterior part of the distal end of the radius is more concave in 

 the Mylodon than in the Megatherium, and the distal articular extremity 

 has its posterior boundary shorter transversely, and more produced 

 downwards. Purchased. 



421. A longitudinal section of the left radius of the Mylodon robustus, showing 



the absence of a medullary cavity : the cancellous texture which occupies 

 the centre of the bone is of somewhat coarser character than in the 

 humerus. Purchased. 



