1 66 THE OSTEOLOGY OF THE REPTILES 



The radius (Figs. 133, 134), on the thumb, radial or preaxial side, 

 articulates with the preaxial condyle of the humerus by a more or 

 less concave and rotating joint, as in the pentedactylate mammals; 

 distally, normally with the radiale of the carpus. The ulna (Figs. 133, 

 134), on the postaxial side, articulates with the trochlear condyle of 

 the humerus, as in mammals, by a hinge, but somewhat spiral joint; 

 distally, normally with the intermedium and ulnare of the carpus, 

 usually also at its distal postaxial angle with the pisiform. In terres- 

 trial reptiles the ulna is produced more or less into an olecranon, or 

 elbow. 



In the aquatic reptiles the two bones, like the posterior epipodials, 

 are shortened, sometimes losing all resemblances to the terrestrial 

 forms. They retain some of their land characters in the early plesio- 

 saurs and ichthyosaurs, but in the more specialized of both groups 

 (Figs. 158 c, D, 159), they are wider than long, articulating with each 

 other throughout their adjacent sides. In some of the later plesio- 

 saurs a third and even a fourth bone, whose homologies are ill under- 

 stood, may articulate with the distal end of the humerus on the 

 postaxial side. A third bone is also known in some ichthyosaurs — 

 an accessory epipodial (Fig. 158 c). 



The radius and ulna of the temnospondylous amphibians (Fig. 

 136) present no characters by which they can be distinguished from 

 those of the Cotylosauria; the olecranon is but feebly or not at all 

 produced. 



Tibia and Fibula. The tibia on the preaxial or big- toe side of the 

 hind-leg is always the larger in terrestrial reptiles (Fig. 135), unlike 

 the radius, which is more often the smaller. It articulates with both 

 condyles of the femur, though chiefly with the preaxial, especially in 

 bipedal forms. Its proximal extremity is expanded into a more or 

 less prominent cnemial crest on the dorsal side for the immediate 

 attachment of the extensor muscles, since there is no patella, and 

 rarely sesamoid bones of any kind, in reptiles. The distal extremity 

 (Figs. 135, 151, 153) articulates exclusively with the astragalus, or 

 the astragalar part of the fused bone. This joint in the early reptiles 

 was extensive and loose, permitting a wide range of lateral movement 

 in the foot; in later reptiles it is closer and firmer. 



The fibula, on the postaxial or little-toe side, is more slender than 

 the tibia in land reptiles. It articulates proximally exclusively with 



