The primitive form of the limb in early synapsid reptiles, ancestral to the therapsid and mammalian line, is illustrated by the skeleton of the left fore limb of the pelycosaur Ophiacodon (Fig. 1). The figure shows several features of mechanical importance: (1) The humerus is heavy, broad, short, and held horizontally from the body; it has a horizontal ridge-like proximal articular surface, two separate distal articular facets, and two greatly expanded epicondyles, of which the ectepicondyle stands erect dorsally and the entepicondyle stands out like a shelf posteriorly. (2) The radius and ulna articulate separately with the humerus, but not at all with each other; their movements were not confined to a single plane (with reference to the humerus) but were (a) extension (laterad) and flexion (mediad), and (b) elevation and depression. (3) The pisiform bone is seen to bear the same relation-