250 THE ARM AND FORE- ARM. [chap. 



is no supracondylar perforation. The ulna is extremely 

 reduced, only the upper third being present, and that anky- 

 losed with the radius, which forms almost the whole of the 

 lower articular surface of the elbow-joint. There is a 

 detached sesamoid ossicle on the olecranon. 



In the RoDENTiA the humerus varies much in its charac- 

 ters. It is long, slender, and straight, with a very slight 

 deltoid ridge, a narrow and laterally compressed inferior 

 end, and without prominent condyles in the Hares and 

 Agutis. But in the Beaver the deltoid and supinator ridges, 

 and the inner condyle, are strongly developed. All inter- 

 mediate conditions occur in different genera. As a general 

 rule there is a large supratrochlear perforation, but no 

 supracondylar foramen. In the Coypu {Myopotamus) the 

 deltoid ridge is an extremely salient, compressed, and 

 everted tuberosity. 



In the fore-arm the two bones are nearly always dis- 

 tinct, though closely applied to each other. The breadth 

 of the upper end of the radius, and the amount of rota- 

 tion permitted upon the ulna, vary much in different 

 genera. 



In the great order Ungulata the humerus is stout and 

 rather short. The outer tuberosity is very large, and gene- 

 rally sends a strong curved process inwards, overhanging the 

 bicipital groove (not, however, in the Horse and Camel). 

 The deltoid ridge is usually not strongly marked, and placed 

 rather high on the bone ; but in the Rhinoceros it is a very 

 salient ridge. The lower end is always particularly straight 

 and flat on the inner side (see Fig. 84, p. 248), the condyle 

 forming no prominence, and there is never a supracondylar 

 foramen. The outer condyle and the ridge above it are 

 rather more developed. 



The radius is large at both ends, and superiorly extends 



