85 



which it presents in its shorter and stronger proportions, as being more suitable 

 to the enormous weight which it assisted in supporting. The more elongated and 

 slender form of the ulna in the Megalonyx might be thought to correspond with 

 the diminished bulk of that extinct Megatherioid quadruped ; but the ulna of the 

 more robust, though smaller genus, Mylodon, has shorter and broader proportions 

 even than in the Megatherium, whilst the olecranon is relatively longer than in any 

 Ant-eater. In the Myrmecophagajubata this process does not equal a fifth part the 

 length of the whole ulna : in the Orycterope it is rather more than a fifth : in the 

 Mylodon the olecranon — measured, as in the above-cited Edentals, from the upper 

 margin of the sigmoid cavity — is more than one-fourth the length of the bone. 

 In the Megatherium it is more than one-fifth the length of the ulna : in the Me- 

 galonyx it is rather less than one-fifth. In the Scelidothere * the olecranon is 

 rather less than one-fourth the length of the ulna, but is longer in proportion to 

 its breadth and thickness than in the Mylodon. The sigmoid articulation of the 

 ulna in the Megatherium and Megalonyx corresponds with the distal articulation 

 of the humerus, and the inner division is consequently more concave than in the 

 Mylodon. The flatness of this part of the tdna in the Scehdothere corresponds 

 with that modification of the distal end of the humerus, in which it resembles 

 the Mylodon. But a greater diff"erence is manifested in the outer division 

 of the sigmoid, or proximal articulation of the uhia in the Megalonyx, and 

 probably also in the Megatherium. When the bones of the fore-arm of the Mylo- 

 don, for example, are viewed in natural juxtaposition, as in Plate XIV. fig. 3, the 

 radial or outer division of the great sigmoid cavity is as large and more concave 

 than the inner one : when the same bones of the Megalonyx are viewed in the 

 same position, the outer half of the sigmoid cavity, above the head of the radius, 

 is much smaller than the inner half, and is convex. Below this there is a large, 

 concave, triangular surface for the radius, divided from the previous convex facet 

 by a groove. 



The shaft of the ulna is relatively longer and more slender in the Megathe- 

 rium than in the Mylodon : its distal extremity presents, at its radial side, a 

 well-developed convexity, which is not present in the Mylodon. The ulna of the 



* Fossil Mammalia of the Beagle, he. cit. p. 91, pi. XXV. fig. 2. 



