The marsupial bones (a, b ) are smaller, rounder, and more curved externally than in the Wombat. Tbeir free extremities are tuber culated and not flattened, and they are articu lated to the pubic crest near the symphysis by a single facet only, the inner, the position of the outer one being marked by a slight tubercle (b). The ilio-pectineal spines (d) are very large, for the attachment of powerful psom muscles.
The direction of the ischio-pubic symphysis (f) in the Kangaroo, Phaseogale, and Potoroo, is not parallel with the spinal column, but oblique in the opposite direction to the human symphysis, so that if prolonged forwards the line of direction would cut the spinal column at an obtuse angle. This makes the posterior opening larger in its antero-posterior diameter than it otherwise would be, and allows for the great encroachment of the caudal ver tebrx posteriorly. The sciatic notch is long and narrow, corresponding to the great length of the ischia ; and the foramen obturatoriuin is large and elongated antero-posteriorly from the same cause. In the Dasyurus and Pe fauvist:, the ischio-pubic symphysis is oblique in the opposite direction. The antero-pos terior diameter of the anterior outlet in the Kangaroo is greater than the transverse by about half an inch ; but at the posterior outlet, the transverse is a little greater, from the pro jection of the caudal spines before mentioned. The pelvic cavity is deep in the Marsupialia, and its.openings are small in proportion to the size of the animal, since the fcetus is expelled before it is full grown, and placed on the nipples in the marsupial pouch to complete its deve lopment into_ a state of independent existence. But the proportion between the pelvic open ings and the size of the fcetal head, at the period of expulsion, is very remote. Even in the Petaurists, whose pelves are the smallest, the cavity and openings are six times the size of the foetal head.
The muscles of the tail and legs attached to the pelvis are, in the Kangaroos, very powerful to perform their prodigious leaps, especially the gracilis and biceps. The glutei, however, are not large, since the trunk is not held erect on the legs by these muscles, but is suspended, as it were, between the femurs, and supported in front by the largely developed ',sow muscles, and behind by the powerful tail, used as a propelling organ by the sudden action of its flexor muscles.
The pelvis of the Monotremata resembles in general appearance the reptile type, although in some other respects these curious animals, especially the Ondthorhyticus, approach the Birds. The sacrum of the Ornithorhyncus is composed of two vertebrm, separated, as in the Saurian reptiles, and placed in the line of the lumbar curve, differing little in appearance from the lumbar vertebrm. In the Echidna are three sacral vertebrw, also separated and all uniting with the ilia.
The ilia are short, thick, and prismatic, and project above the spine at an angle of 140° as high as the sacral spines, and presenting, in the Ornithorhyncut, considerable eversion of the aim, and, in a much less degree, in the Echidna also. The ischia are short, bent upwards in the former, and project backwards at the tuberosi ties in an angular spine, most marked in the Ornithorhyncus, and giving a reptile-like ap pearance. The pubes are broad and short, placed at a marked angle with the ilia, 110° in the Echidna hystriv and 120° in the Oriiitho rhyneus, and uniting by broad plate-like rami with the ischial rami, which forin with them a long ischio-pubic symphysis. The ischio pubic plate thus formed is very like that seen in the reptiles.
The marsupial bones are also present, and are very large and strong in this cla.ss, although not provided with a pouch. In the Ornitho rhyncus they are broad and triangular, articu lated by the base to the whole length of the pubic crest meeting in the median line, and with their rounded apices directed forwards and outwards. In the Echidna they are longer, rounder, more pointed and less everted, with two articular processes at the pubic extremity (see fig. 177. Art. Itionotremata).
The ilio-pectineal spines are also very large in the Ornithorhyncus, and in a less degree in the Echidna. The obturator foramina are small. The 'three pelvic bones are united at the cotyloids by bony union in the Ornithorhyncus. In the Echidna hystris, the union of these bones is, however, effected by cartilage only, and the acetabula are perforated by a consider able opening into the pelvic cavity, constituting another remarkable reptile.like peculiarity.