THE PECTORAL AND PELVIC GIRDLES 



151 



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 



Fig. 123. 'PtW\s oi Platecarpus (Mosasauria), from below. 



Fig. 124. Pelvis of A^oMoJ««r«j (Nothosauria). After Andrews. 



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 sym- 

 physis, and there is a pubic foramen. 



In the earlier ichthyosaurs the broad ischia and pubes were sepa- 

 rated by the broad pubo-ischiatic opening, and the pelvis was con- 

 nected with a sacrum. In the later forms, however, the pelvis was re- 

 duced, 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. 



