ii6 



THE OSTEOLOGY OF THE REPTILES. 



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 re- 

 main in the pterodactyls and turtles; they are nearly always fused. 

 Three or four of the single-headed ribs of the Sauropterygia articu- 

 lating 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 verte- 

 brae in front of the sacrum bear no ribs of any kind; in others, 

 Seymouria (Fig. i) for instance, free ribs continue to the sacrum. 

 Many other reptiles have a variable number of the presacral ribs 

 coossified with the centrum, or centrum and arch, so-called trans- 

 verse processes. 



Sacral ribs. True sacral ribs often retain their primitive attach- 

 ments (Fig. 93), the capitular part articulating more or less inter- 

 centrally with the preceding vertebra, the tubercular part with the 



