Pinnipedia, _Manis_ among Edentates, the Cetacea, Sirenia, Ungulates, and some Rodents. The Monotremes are peculiar in possessing a T-shaped interclavicle like that of many reptiles, lying upon the sternum, and articulating superiorly with the clavicles. _Brachium and Antebrachium._—The proximal segment of the anterior or pectoral limb proper contains a single bone, the humerus, and the second segment two bones, the radius and the ulna, placed side by side, and articulating with the humerus at their proximal, and with the carpus at their distal extremity (Fig. 15). In their primitive and unmodified condition these bones may be considered as placed one on each border of the limb, the radius being preaxial or anterior, and the ulna postaxial or posterior, when the distal or free end of the limb is directed outwards, or away from the trunk. This is their position in the earliest embryonic condition, and is best illustrated among adult mammals in the Cetacea, where the two bones are fixed side by side and parallel to each other. In the greater number of mammals the bones assume a very modified and adaptive position, usually crossing each other in the forearm, the radius in front of the ulna, so that the preaxial bone (radius), though external (in the ordinary position of the limb) at the upper end, is internal at the lower end; and the hand, being mainly fixed to the radius, also has its preaxial border internal. In the large majority of mammals the bones are fixed in this position, but in some few, as in Man, a free movement of crossing and uncrossing—or pronation and supination, as it is termed—is allowed between them, so that they can be placed in their primitive parallel condition, when the hand (which moves with the radius) is said to be supine, or they may be crossed, when the hand is said to be prone. The humerus frequently has a foramen piercing the inner border of the distal extremity, known as the entepicondylar foramen, which corresponds with a similar one found in the Anomodont reptiles. The hollow in the head of the ulna for the reception of the head of the humerus is known as the greater sigmoid cavity, and that for the head of the radius as the lesser sigmoid cavity (Fig. 16). The term olecranon is applied to that process of the ulna which forms the prominence of the elbow. [Illustration: FIG. 16.—Outer aspect of the proximal extremity of the right ulna of a Bear (_Ursus_). _a_, Anterior tubercle; _ol_, olecranon; _b_, greater sigmoid cavity; _c_, lesser do.]