In the Allarsupialia the pelvis is also much developed, both to afford attachment to the powerful muscles of the tail and hinder legs, and, in some of the order, to support the ab dominal viscera in their sitting posture and leaping movements. It is remarkable for the development, in most of the genera, of two extra bones (Ar. 99. a.) characteristic of the order, which are attached to the pubes in the site of the crests and spines of these bones in other animals, and support the abdominal pouch peculiar to them and destined for the reception and more mature development of their young, expelled immature from the pelvic cavity. They are directed obliquely forwards and a little outwards, in the direction of the fibres of the aponeurosis of the external oblique niuscle of the abdomen ; in which, according to Owen and Laurent, these bones are de veloped by ossification of the fibrous tissue. The free extremities are a little curved, and over them the cremaster muscles in the male anitnals play. The pelves of the Marsupials differ considerably, according as the mode of progression is quadrupedal, or by a succession of springs from the tail and strong hinder ex tremities. The Wombat may be taken as an example of the former, and the Kangaroo of the latter pelvic type, which approaches in general form to the pelvis of the Jerboa, just considered.
The sacrum of the Wombat is very flat, and strong, and broad, in correspondence with the general squat and massy skeleton of the animal (see figs. 102. and 108. Art. Marsu pialia). Its curve is a continuation of that of the dorsal and lumbar vertebrm. The number of sacral vertebrm is seven, and the transverse processes are separated from each other, the three upper of which are long and strong, and are articulated by their tips to the ilia. The facet on the extremity of the first looks upward and outward, and that on the second, on the contrary, downward and outward, and form projections which impinge upon the iliac facet. This arrangement in the Wombat like that in the Tapir, is closely ana logous to the formation of the sacral " joggles," and the alteration of the inclination of the sacral wedge in Man, at the point of the auri cular surface opposite the second sacral piece, substituting for upward, backwards, and for downward, forwards, a change consequent on the difference between the prone and erect positions. Caudal pieces are numerous.
The ilia are comparatively short and ex panded considerably, and are curved out wards in a remarkably strong, broad, hook like process at the anterior superior spine. They are placed very obliquely on the spine, being at an angle of 160° with the lumbar portion of the great dorsal curvature. The ischia are thick, long, and massy, and in a right line with the ilia. They have cu riously bifurcated tuberosities, one tubercular projection turning inward, and the other longer, curving outward, in another remark able and strong hook-like process, to which formation we have before remarked a tendency in the Hare and other Rodents (fig. 98. e). These processes, like those on the ilia, afford a powerful hold and leverage to the strong muscles of the hinder extremities, much used by the animal in its burrowing habits. These hook-like processes of the ischia are formed by a Y-shaped apophysis analogous to that of the tuberosity in man. The pubes are short
and thick ; and the symphysis is parallel to the spine, very long, and joined in very ex tensively by the vertically directed rami of the ischia.
The 2narsupial bones of the Wombat are long, flat, rounded and expanded at their free extre mities, and articulated to the anterior border of the pubes in the position of the crests by two articular facets separated by an arched interval. Ilio-pectineal spines are present, and of large size.
The whole pelvis of the Wombat is large and massy, though the openings and outlets are proportionably very small. It has a flattened appearance antero-posteriorly, so that the anterior outlet has its greatest diameter trans versely placed.
In the Myrmeeobius jitsciatus this flattened appearance of the pelvis is still more remark able.
In the Opossums, Peranwles, and Pha langers, there is but one sacral vertebra, which, in the Phalangista Cookii, is ankylosed to the last lumbar vertebra.
The pelvis of the Thylacinus Cynocephalus approaches closely in many respects to the type of the Carnivora, like many of the peculiarities of this animal. The sacrum pre sents no angle ; the ilia are massy, somewhat short, and less oblique than those of the Wombat ; the ischia are also short and thick, and are placed at an open angle (170°) with the ilia, while the pubes are short and di rected almost horizontally inwards, making an antero-posterior angle with the ilia as little as 115°. The whole appearance of the pelvis is massy, with small openings. It has neither the oblique appearance and exaggerated pro cesses of the Wombat, nor the elongation and wide outlets of the Kangaroo, while its well marked ilio-pubic angle contrasts much with both, and shows a strong similarity to the Carnivora.
In the Kangaroos, the sacrum is in the line of the lumbar curve, and differs in little but size and breadth from the preceding vertebrm. There are two sacral vertebrm, articulating with the ilia, their transverse processes being long and coalesced, but the spinous processes distinct. In the Potoroo there is one only, with large lateral processes. The caudal vertebrm are numerous and very strong, and their upper normal spines encroach much on the diameter of the pelvic cavity and posterior outlet. The ilia have short, strong, and pris matic shafts as in the Rodents, with alm of the same shape, much elongated and turned out wards, though in a much less degree than in the Wombat, and terminating in narrow clubbed crests (fig. 99. g). There is a rudimentary inferior anterior spine (h). The upper part of the iliac wing projects much on the dorsal aspect of the spinal column, forming with it an angle of 140° (see fig. 112. 9.). The Ischia are very long, broad, and strong, and have much-expanded tuberosities with an outward curvature (fig. 99. e). These are united in a median symphysis by a single V-shaped epi physis (c), divided, in the adult, uy a suture from the ischia. The tuberosities support also another epiphysis on each side posteriorly at e, the anterior ischial rami being almost deficient. The ilia and ischia are very nearly in a direct line. The pubes are moderately long, slender, and directed much downward, so as to give to the anterior outlet a triangular shape, with the base at the broad sacrum, and the apex at the pubic symphysis. The ilio-pubic angle is 135°.