252 THE ARM AND FORE-ARM. [chap. 



developed olecranon process projecting directly outwards 

 from the shaft of the bone ; the radius is extremely simple 

 in form, wider below than above. 



In the Rorquals (Balmnoptera and Megapterd) these 

 bones are considerably elongated. 



In the Sirenia the bones of the fore-limb are formed on 

 a different type, as there is a distinct, though small and 

 simple, trochlear articulation at the elbow-joint. In the 

 Dugong, the humerus is small in the middle of the shaft, 

 and expanded at each end. The tuberosities are very pro- 

 minent, especially the outer one, and the bicipital groove is 

 distinct. The internal condyle is prominent, the anconeal 

 fossa small, and there is no supracondylar perforation. In 

 the humerus of the Manati the bicipital groove is obsolete, 

 the two tuberosities coalescing, as in the Cetacea. In other 

 respects it resembles that of the Dugong. 



The two bones of the fore-arm are, in both genera, about 

 equally developed, and generally ankylose together at both 

 extremities. 



Order Edentata. — In the Sloths the humerus is long 

 and straight, slender and cylindrical in the greater part, but 

 flattened and laterally expanded at the lower end. The 

 head is hemispherical, the tuberosities moderately developed, 

 and subequal in size, the deltoid ridge very indistinct. In 

 the Two-toed Sloths (Cholcepus) the humerus is shorter and 

 broader than in JBradypus, and has a large supracondylar 

 perforation, which is wanting in the latter genus. 



The radius and ulna somewhat recall those of the 

 Primates in their form, and they are capable of a con- 

 siderable amount of pronation and supination. The 

 olecranon process scarcely projects beyond the sigmoid 

 articular surface. 



The humerus in all the remaining Edentates is stout and 



