71 



respect from that of the Megalonyx, in the absence of the perforation of 

 the internal condyle, which in the Mylodon is simply notched at its upper 

 part by the brachial nerve and vessels as in the Megatherium ; but the 

 notch is less strongly marked in the more gigantic species. 



The form of the articular surface for the ulna offers another well- 

 marked distinction between the Mylodon and Megalonyx. In the latter 

 species, and likewise, to judge from the figures of Bru and Pander, in the 

 Megatherium, this part of the distal articular surface of the humerus is 

 convex in every direction : in the Mylodon it is only convex, and that in 

 a very slight degree, from before backwards, and is concave from side to 

 side. Purchased. 



414. A longitudinal section of the left humerus of the Mylodon robustus: it 



shows the course of the canal for the medullary artery, which enters the 

 bone a little way above the inner condyloid plate, and passes slightly 

 upwards in a straight line to an oblong cell four lines in diameter, 

 which is the sole rudiment of the medullary cavity in this bone. The 

 compact wall of the humerus is thickest near its middle part where the 

 medullary canal exists ; it diminishes to a depth of one or two lines at 

 the extremities of the bone, the included substance presenting a nearly 

 uniform and close cancellous texture. Purchased. 



415. The opposite section of the same bone. Purchased. 



416. A portion of the proximal half of the right ulna of the Mylodon robustus, 



showing the smaller sigmoid cavity for the head of the radius. 



Purchased. 



41/. The proximal half of the left ulna of the Mylodon robustus, showing the 

 slightly concave surface adapted to the inner condyle of the humerus. 



The olecranon is bent obliquely inwards ; the broad and rough back 

 part of the olecranon gradually contracts into the posterior flattened 

 border of the ulna. The great sigmoid or rather reniform articular sur- 

 face extends almost transversely across the base of the olecranon, and 

 plays upon the inner and back part of the outer condyle of the humerus, 

 being divided by a median convexity into two compartments; the inner 



