short and more or less "hatchet-shaped," either fused or more or less closely united to arch and centrum. The free cervical ribs of lizards and mosasaurs begin upon the axis. Only vestiges of ribs remain in the pterodactyls and turtles; they are nearly always fused. Three or four of the single-headed ribs of the Sauropterygia articulating with both centrum and arch are known as pectoral ribs.
Fig. 91. Inner side of carapace of Stegochelys (Chelonia). After Jaekel.
About one sixth natural size.
In certain early cotylosaurs (Figs. 128, 164), four or five vertebrae in front of the sacrum bear no ribs of any kind; in others, Seymouria (Fig. 1) for instance, free ribs continue to the sacrum. Many other reptiles have a variable number of the presacral ribs coössified with the centrum, or centrum and arch, so-called transverse processes.
Sacral ribs. True sacral ribs often retain their primitive attachments (Fig. 93), the capitular part articulating more or less intercentrally with the preceding vertebra, the tubercular part with the
