The posterior and external angle of the ischium is rough and prominent; it is the tuberosity of the ischium. This forms a projection under the skin; it also does in man when the trunk is strongly inclined forwards, while the thighs are maintained in the vertical position. In marsupials--opossum, kangaroo, and phalanger--the pelvis at its pubic region is surmounted by two bones, situated one on each side of the median line, and arranged in the form of a fork of two prongs (Fig. 45). These, which are called _marsupial bones_, support the pouch which, in animals of this genus, lodges their young, which, at the time of birth, are incapable of supporting a separate existence, their development being absolutely incomplete. [Illustration: FIG. 45.--PUBIC REGION OF THE PELVIS OF A MARSUPIAL (PHALANGER, FOX). 1, Symphysis pubis; 2, obturator foramen; 3, marsupial bones.] In the cetaceans--for example, the dolphin--because of the absence of posterior limbs, the pelvis is represented by two separate bones only, which have no connection with the vertebral column. In birds, the pelvis is remarkable for its elongated form (see for its form Fig. 21, and for details Fig. 46). The cotyloid cavity is pierced by an opening, and presents on its posterior border, which is here a little prominent, a surface with which the great trochanter is in contact. The ilium is very highly developed, and is fused in the median line with the ilium of the opposite side, the last dorsal vertebræ, the lumbar vertebræ, and the sacrum. Because of these relations with the dorsal vertebræ, it is in contact anteriorly with the last ribs, which consequently emerge from each side of the iliac region of the pelvis. The ischium forms a plate of bone which, in part, closes the external portion of the cavity of the pelvis. Its superior border is separate for a certain distance from the external border of the ilium; there is thus left an opening of more or less considerable size, which represents or takes the place of the great sciatic notch. [Illustration: FIG. 46.--PELVIS OF A BIRD (THE COCK): EXTERNAL SURFACE, LEFT SIDE. 1, Ilium; 2, ischium; 3, pubis; 4, inferior extremity of the pubis; 5, sciatic foramen; 6, oval foramen, homologous to the obturator; 7, coccygeal vertebræ.] The pubis, long and slender, is in connection with the inferior border of the ischium, of which it follows the general direction; and circumscribes with this latter, below the cotyloid cavity, an oval orifice, which is the homologue of the obturator foramen. Its inferior extremity reaches beyond the corresponding part of the ischium, bending towards the middle line, but without joining the pubis of the opposite side. On this account there is no symphysis pubis in birds. Nevertheless, an exception must be noted in the case of the ostrich, the pubic bones of which meet in the middle line, and are articulated in form of a symphysis. The Thigh