Page:The Osteology of the Reptiles.pdf/138

120 and Theropoda, of the Archosauria; in the Sauropterygia, Ichthyosauria, 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.



. 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 ), 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