Page:The Osteology of the Reptiles.pdf/169

Rh due to differences in procreational methods, the open pelvis of the predentates permitting larger eggs to be extruded, as in the birds. It may be added that the acetabulum of all dinosaurs is perforate, as in the birds.

The pelvis of amphibious or aquatic reptiles is also modified not a little. It lost all connection with the spinal column in the Mosasauria and later Ichthyosauria, but is firmly connected, as usual, in other water animals. The slender ilium of the mosasaurs, like that of the ichthyosaurs, lay loosely in the flesh with its upper end in apposition or ligamentously connected with the end of a transverse process or rib of a single vertebra. The narrow ischia and pubes meet in a symphysis, and there is a pubic foramen.





In the earlier ichthyosaurs the broad ischia and pubes were separated by the broad pubo-ischiatic opening, and the pelvis was connected with a sacrum. In the later forms, however, the pelvis was reduced, the rod-like ilium lay loosely in the flesh, and the pubes and ischia were united without a pubo-ischiatic opening.

In the Nothosauria (Fig. 124) the pelvis, of the usual type, shows only a moderate aquatic adaptation in the broad pubes and ischia.