492 MAMMALIA. 



humerus often has a foramen above the inner condyle called the 

 entepicondylar foramen. Its presence is supposed to be a 

 primitive feature, as it is chiefly found in the lower and older 

 (extinct) forms. It is characteristic of some ancient reptiles 

 e.g. Hatteria, Anomodontia, etc. The radius and ulna in the 

 fore-limb and the tibia and fibula in the hind-limb are almost 

 always longer than the humerus and femur respectively. The 

 ulna forms the hinge-joint of the elbow, and is prolonged at this 

 point into a process called the olecranon ; the radius, on the 

 other hand, is connected with the carpus, and can often be 

 rotated round the ulna (pronation, supination) ; in other cases 

 it is fused with the ulna, which then constitutes a rudimentary, 

 styliform rod continued from the olecranon process. In the 

 hind-limb the knee-joint projects forwards, and is usually 

 covered by a knee-cap, the patella ; the fibula is sometimes 

 (marsuj)ials) moveable on the tibia, but as a rule these two bones 

 are fused, and the fibula which is placed posteriorly and ex- 

 ternally is usually reduced. 



Tlie variations in the terminal parts of the limbs are far 

 more striking (Fig. 262). The number of digits is never greater 

 than five, and is often less. The order of their disappearance 

 has already been described (p. 480). 



In the carpus of Mammalia the three proximal bones remain 

 distinct, a centrale is occasionally present {Hyrax, many Insecti- 

 vora. Primates, etc.) and carpalia 4 and 5 of the distal row are 

 always fused together. There is always a radial sesamoid de- 

 veloped in the tendon of the flexor muscles and called the pisi- 

 form. There may also be a smaller ulnar sesamoid. The names 

 given to these bones are shown in the following table, with the 

 equivalent terms used by Comparative Anatomists. 



