THE RIBS AND STERNUM 123 



cestors never possessed it. There could have been no sternum, even a 

 small cartilaginous one, among the Cotylosauria and Theromorpha, 

 since in several instances in both these orders the parasternals have 

 been found sheathing the whole abdomen from the coracoids to the 

 pelvis. Probably this was the condition in all the early reptiles; 

 probably also the condition in the early Rhynchocephalia, since 

 Rhynchosaurus had parasternal ribs reaching nearly to the coracoids, 

 leaving little or no space for a sternum. 



In the modern Lacertilia (Fig. 99) and in Sphenodon there is a 

 more or less calcified, thin, rhomboidal plate articulating on each side 

 with the coracoid in front and ending in a single or paired continu- 

 ation, the xiphisternum. It gives articulation on each side to from 

 one to four or five, exceptionally more, sternal ribs, also cartilagi- 

 nous. Rarely, no ribs articulate with the sternum, and the sternum 

 itself may be represented by a pair of small cartilaginous plates or 

 may be wholly absent. Sometimes it has single or paired perfora- 

 tions. Similar cartilaginous sterna have been found in the DoH- 

 chosauria and Mosasauria, and doubtless it was present in most of 

 the extinct members of the order. 



In the Chelonia there is no trace of a sternum. In the hving 

 Crocodilia the sternum is a small, oval, cartilaginous plate, contin- 

 ued into a pair of cartilaginous xiphisternal rods to which six or eight 

 dorsal ribs are attached by the intervention of cartilaginous sternal 

 ribs. Nothing is known of the sternum in extinct crocodiles or 

 phytosaurs; probably it was present as a cartilage. 



The sternum has been found in not a few dinosaurs. Among the 

 Sauropoda it has been recognized in a pair of oval, ossified plates. 



