Page:The Osteology of the Reptiles.pdf/172

154 ischium only, and, like that of the Chelonia, is directed upward and backward. The pubes and ischia, like the coracoids, are very broad and flat, secondarily plate-like, meeting in a more or less horizontal symphysis. There is no pubic foramen, and usually the large pubo-ischiatic vacuity is broadly connected across the median line—probably separated by a ligament in life. In some genera, however, Sthenarosaurus or Thaumatosaurus, for instance, the two bones are secondarily broadly united at their symphyses, producing a false thyroid foramen with which the obturator foramen is confluent, as in mammals. The ischia are triangular or "hatchet-shaped," elongated in the short-necked forms, short in the long-necked.

The pelvis of the Chelonia (Fig. 127), like the pectoral girdle, has been modified by its peculiar relations to the carapace and plastron. There is a large pubo-ischiatic vacuity, often divided in the middle by a cartilaginous septum, but broadly ossified in the land tortoises, as in the plesiosaurian Sthenarosaurus. As in the plesiosaurs, there is no separate pubic foramen or notch, rarely absent in reptiles.

The ilium, like that of the plesiosaurs, is elongate and is directed upward and backward to the firm sacrum. The pubis is larger than the ischium and has a stout tuberosity which rests upon the plastron, or, in the Pleurodira, is coössified with it.

Usually in crawling reptiles (Figs. 114–118 ) there is no, or only a small, preacetabular process to the ilium, but always a postacetabular one. In upright-walking animals the preacetabular process is always well developed, sometimes at the entire expense of the postacetabular process. It is unusually long in the Anomodontia (Figs. 120, 119), Ceratopsia (Fig. 122 ), and Pterosauria (Fig. 118 ), where it is supported by the united or contiguous diapophyses of the lumbar vertebrae, false sacral vertebrae. The ilium is more or less helmet-shaped in the Saurischia (Fig. 122 ) as also in some Cotylosauria, Therapsida (Cynognathus), and Theromorpha (Casea)—all such forms have short toes; possibly it is due to the greater expansion of the gluteal muscles.

The evolution of the reptilian pelvis has been, as we have seen, from the primitive closed and plate-like type, by the progressive development of a vacuity between the ischia and pubes, by the elongation of the anterior process of the ilium, and by its closer union with additional true sacral or lumbar vertebrae.