The pelvis of the Cursores (fig. 106.) ap proaches most nearly in the massiveness of the bones to the Mammalian type, as well as, in the Ostrich, in the formation of a pubic symphysis.
The sacrum (a) is very long and narrow, and is composed, according to Cuvier's tables, of twenty pieces in the Ostrich and of nineteen in the Emu and Australian Casso wary. The spinous and fransverse processes are distinct, and coalesced only at their ex tremities (a), in the Ostrich (the former being the only part of the sacrum appearing dorsally), presenting another close approximation to the Mammalian condition.
The coccyx is straight, and composed of seven pieces, which are perforated for the terinination of the spinal marrow, and end in a conical bone. In the Rhea or American Ostrich, both the sacral and coecygeal bones are much atrophied.
The ilia (b) are comparatively very short, especially in the Rhea. The aim are thick, short, and little curved, and lie close to each other at the upper half of their inner borders, by which they are ankylosed to tbe sacral spines, whose coalesced extremities are seen between them, forming a tent-like eminence above the anterior sacral vertebr, and sup ported by their spinous processes in the manner of tent poles. Anteriorly, they overlap the posterior ribs ; and posteriorly, they are prolonged on the sides of the sacrum into a distinct and prominent ilio.sacral element.
This is an elonoated piece of bone, with a superior (c) :id a lateral (c') surface, tapering gradually to the posterior extremity of the sacrum, where it terminates in an outward curve ; and placed upon the transverse processes, which it encases, like a frame, on each side, and to which it is firmly anky losed by its inner surface. In the Ostrich, its thick upper surface or border does not unite with the similar bone of the opposite side, nor with the sacral crest; but is separated from it by a chink,-or oval opening, d, gradually narrowing posteriorly, in which the sacral spines (a) are seen distinct and separate, and coalesced only at their extremities. Opposite the three last sacral spines, however, the ilio-sacral pieces are ankylosed to the sacral ridge, and terminate posteriorly the oval chink. In the Etnu and Rhea, the ilio-sacral pieces are coalesced in their whole length with the extremities of the sacral spines, and a narrow diamond-shaped dorsal plate is formed, composed almost entirely of the united ilio sacral plates, and having its angles at the massy ischio-sacral buttresses.
The ischia (e) are very long and thick, and form, by the superior vertical bifurcation of the cotyloid extremity, a very strong ischio sacral buttress (f), coalescing at that point with the ilium, sacrum, and ilio-sacral plate. In the Ostrich and Emu the ischia are not connected posteriorly with the sacrum, but a wide and elongated sacro-sciatic notch inter venes. In the latter, the ischial extremities are free and tubercular. In the Rhea they are ankylosed, by their posterior four-fifths, not only to the sacrum, but, like the ischia of the Bats, to each other, passing in front of the coccyx and greater part of the sacrum, thus excluding them from the pelvic cavity, and enclosing complete sacro-sciaticforamina,which open into a sort of posterior pelvic cavity.
The Tubes (g) are long and slender, and are united posteriorly to the ischia in the Ostrich and Rhea, completing the Obturator Foramina ; but, in the Cassowary, the pubes, as well as the ischia, are free at their posterior extremities, and the obturator foramina are incomplete, like the sacro-sciatic. In the Rhea and Cassowary they are widely diverging ; but in the ostrich they approach each other in a wide curve posteriorly, and unite in a median interpubic symphysis (h), which curves forward anteriorly in a hook-like process, and completes an oblong anterior pelvic outlet with its longest diameter antero-posterior. The ilio-pubic angle is 140° in the Rhea, and 155° in the Ostrich and Cassowary. The ilio pectineal spines are well marked, especially in the Ostrich (m). The acetabula (i) are per forated, and open partly into the pelvic cavity, and partly upon the sacrum, and are so closely approximated that the bodies of the vertebrm only intervene. Immediately below the ace tabula, the ischia and pubes are connected, on each side, by the suture of an ischial apophysis with the pubes (k), across the obturator mem brane, enclosing a stnaller obturator opening (/), which transmits the vessels and nerves, and intervenes between the larger obturator opening (o) and the acetabulum.