_Pelvic Girdle._—The pelvic girdle is present in some form in all mammals, though in the Cetacea and the Sirenia it is in an exceedingly rudimentary condition. In all mammals except those belonging to the two orders just named, each lateral half of the pelvic girdle consists essentially, like the corresponding part of the anterior limb, of a flattened rod of bone crossing the long axis of the trunk, having an upper or dorsal and a lower or ventral end. The upper end diverges from that of the opposite side, but the lower end approaches, and, in most cases, meets it, forming a symphysis, without the intervention of any bone corresponding to the sternum. The pelvic girdle differs from the shoulder girdle in being firmly articulated to the vertebral column, thus giving greater power to the hinder limb in its function of supporting and propelling the body. Like the shoulder girdle, it bears on its outer side, near the middle, a cup-shaped articular cavity (“acetabulum”), into which the proximal end of the first bone of the limb proper is received. Each lateral half of the girdle is called the “os innominatum,” or innominate bone, and consists originally of three bones which unite at the acetabulum. The “ilium” or upper bone is that which articulates with the sacral vertebræ. Of the two lower bones the anterior or “pubis” unites with its fellow of the other side at the symphysis; the posterior is the “ischium.” These lower elements form two bars of bone, united above and below, but leaving a space between them in the middle, filled only by membrane, and called the “thyroid” or “obturator” foramen. The whole circle of bone formed by the two innominate bones and the sacrum is called the pelvis. In the Monotremata and Marsupialia, a pair of thin, flat, elongated ossifications called epipubic or marsupial bones are attached to the fore part of the pubis, and project forward into the muscular wall of the abdomen.