646 



REPTILIA. 



RELATIONSHIPS 



Reptiles, in their widest sense, form a central assemblage 

 among Vertebrates. Some of the extinct forms exhibit 

 affinities with Amphibians, others with Birds, others again 

 with Mammals. Though we cannot with certainty point to 

 any of the extinct types as directly ancestral to Birds or 

 Mammals, it seems likely that the ancestors of both were 

 derived from the plastic Reptilian stock. 



N.SP 



CSC 



R 



N,5P 



H 



CA 



FIG. 353A. Vertical section through backbone and 

 ribs of Chelonian (I.) and Mammal (II.). In 

 part after Jaekel. 



.) neural spine ; N.SC., neural scute ; 7 1 ., tubercle of rib ; 

 C.SC., costal scute over rib G.) ; CA., capitulum of rib ; 

 T.P., transverse process; CE. t centrum; NA., cavity of 

 neural canal. In the Chelonian the tubercle abuts against 

 the flattened neural spine, a'nd the capitulum against the 

 transverse process. In the Mammal, the tubercle articu- 

 lates with the transverse process and the capitulum with 

 the centrum. 



