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 continuation, the xiphisternum. It gives articulation on each side to from one to four or five, exceptionally more, sternal ribs, also cartilaginous. 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 perforations. Similar cartilaginous sterna have been found in the Dolichosauria 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 living Crocodilia the sternum is a small, oval, cartilaginous plate, continued 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.
