SKELETON IN MAMMALIA. UPPER ARM AND FORE-ARM. 501 



and ulna can be rotated on one another to a considerable 

 extent. The humerus in all other Edentates is very strong and 

 has the points for the attachment of muscles much developed, 

 especially in the Armadillos and Megatheriidae. An ent- 

 epicondylar foramen is found in all living forms. The radius 

 and ulna are well developed, but are not capable of much 

 rotation. 



In the SIRENIA the humerus is well developed and of a 

 normal character. It is expanded at each end and has a 

 prominent internal condyle, a small olecranon fossa, and no 

 ent-epicondylar foramen. In the Dugong and Rhytina there 

 is a bicipital groove and the tuberosities are distinct, but in 

 the Manatee there is no bicipital groove, and the tuberosi- 

 ties coalesce. The radius and ulna are about equally developed 

 and ankylosed together at both ends. 



In the CETACEA the arm bones are very short and thick. 

 The humerus has a globular head, and a distal end terminated 

 by two equal flattened surfaces to which the radius and ulna 

 are united. There is no bicipital groove, and the tuber- 

 osities coalesce. The radius and ulna are flat expanded bones 

 fixed parallel to one another, but the ulna has a definite ole- 

 cranon. Scarcely any movement can take place between 

 them and the humerus, and in old animals the three bones are 

 often ankylosed together. 



In the UNGULATA VERA the humerus is stout and rather 

 short. The great tuberosity is always large and often overhangs 

 the bicipital groove, it is especially large in Titanotherium 

 (Brontops], There is never an ent-epicondylar foramen. The 

 radius is always large at both ends, but the condition of the 

 ulna is very variable. Sometimes, as in Tapirus, Rhinoceros, 

 Macrauchenia, Suina and Tragulina, the ulna is well developed, 

 and quite distinct from the radius ; but in most forms, although 

 complete, it is much reduced distally, and is fused to the 

 radius. Sometimes, as in the Horse and Giraffe, it is reduced 

 to the olecranon and to a very slender descending process 



