THE VERTEBRAE 



107 



ing a sacrum the number between 

 the girdles may be much greater, 

 thirty-five in the mosasaurs, and 

 as many as seventy-four in some 

 terrestrial, legless lizards. 



As has been said, there is not 

 often the same distinction between 

 thoracic and lumbar vertebrae 

 that there is in mammals. There 

 are, however, even in the Coty- 

 losauria, examples (Fig. 164) of 

 true lumbar vertebrae, that is, 

 vertebrae in front of the sacrum 

 not bearing ribs of any kind. 



Sacral Vertebrae 



(Fig. 83) 



The sacrum of land vertebrates 

 is composed of from one to four 

 or five vertebrae, either fused to- 

 gether or separate, bearing short, 

 stout ribs for the support of the 

 pelvis. Rarely among the am- 

 phibians are there more than one ; 

 certain temnospondyls and mod- 

 ern urodeles^ are known to have 

 two. It is quite certain, however, 

 that reptiles began their career 

 with but a single rib-bearing sacral 

 vertebra, inasmuch as Seymouria 

 of the Cotylosauria is known to 

 have no more (Fig. i). A second 

 vertebra (Fig. 84), however, was 

 soon added from the basal caudal 

 series by the enlargement of the 

 ribs to come in contact with the 

 ilium on each side. And this num- 



^ [Also some frogs. — Ed.] 



Sri 



Fig. 83. Sacrum and caudal vertebrae of 

 Macrochelys (Chelonia), seen from below. 



