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 
Fic. 3534.—Vertical section through backbone and 
ribs of Chelonian (I.) and Mammal (II.).—In 
part after Jaekel. 
N.SP., neural spine; NV.SC., neural scute; 7., tubercle of rib; 
C.SC., costal scute over rib (7.); CA., capitulum of rib; 
7.P., transverse process; CZ., centrum; WVA., cavity of 
neural canal. In the Chelonian the tubercle abuts against 
the flattened neural spine, and the capitulum against the 
transverse process. In the Mammal, the tubercle articu- 
lates with the transverse process and the capitulum with 
the centrum. 
