I20 



THE OSTEOLOGY OF THE REPTILES 



and Theropoda, of the Archosauria; in the Sauropterygia, Ichthyo- 

 sauria, and Chelonia. They are thus, it is seen, characteristic of the 

 Reptilia as a whole, though frequently absent in forms related to 

 those which possess them. No explanation has yet been given of 

 their inconstancy. 



Fig. 94. Sternum and parasternum: A, Theropleura (Theromorpha). About one half natural 

 size. B, Sphenodon (Rhynchocephalia). Three halves natural size. C, Champsosaurus 

 (Choristodera). One half natural size. D, Lystrosaurus (Anomodontia). One half natural 

 size. E, Nyctosaurus (Pterosauria). Nearly one half natural size. 



The most primitive parasternals known among reptiles are those 

 of the Cotylosauria and Theromorpha (Fig. 94 a), slender, bony 

 rods composed of several pieces on each side, meeting in a median, 

 unpaired, V-shaped piece. They sheath closely the whole under side 

 of the abdomen from the coracoids to the pelvis, twelve to fifteen 

 times as numerous as the overlying vertebrae, and more than two 



