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 terrestrial 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 plesiosaurs 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 plesiosaurs a third and even a fourth bone, whose homologies are ill understood, 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
