Page:The Osteology of the Reptiles.pdf/178

160 radial or lateral tuberosity or process. On the opposite side, nearer the head, and often not well marked, is the ulnar or medial tuberosity (Figs. 129, 130). Between the two, on the ventral side, is the bicipital fossa (Fig. 131). Immediately below the lateral tuberosity the shaft is usually round or oval in cross-section. Among all reptiles the lateral process is most developed in the pterodactyls (Fig. 141). It is also largely developed in the Cotylosauria (Figs. 128, 130, 133), Theromorpha (Figs. 129, 131, 134), and Anomodontia, sometimes descending below the middle of the bone.



. 131. Ophiacodon mirus Marsh (Theromorpha). A, left humerus, ventral side, one half natural size. B, left humerus, distal end, one half natural size. C, left ulna, radius and carpus, ventral side, one half natural size. D, left carpus, dorsal side, three fourths natural size.

The expanded extremities of the humerus are in divergent planes, the angle sometimes slight, at other times approximating or even exceeding a right angle, the bicipital fossa in such cases looking more dorsad than ventrad. The width of the more expanded distal extremity may be less than an eighth of the length of the bone, or may nearly equal it in stout-limbed reptiles like the Cotylosauria (Figs. 130, 133). The distal expansion is always great in the Cotylosauria and Anomodontia, as also in some Theromorpha (Figs. 129, 131). Doubtless in these animals, or some of them at least, the peculiar humerus is to be correlated with the screw-like motion in the glenoid