THE SKELETON OF REPTILES 



37 



The breast bone or sternum, while not properly a part of the 

 pectoral girdle, may be mentioned here. In reptiles it is rarely 

 well developed or even ossified, the flying reptiles known as the 

 pterodactyls being the most notable exceptions. It was a com- 

 paratively late developrnent in this class, the earUest ones not 

 possessing it even in a cartilaginous condition. It was doubtless 

 evolved from the more or less numerous and slender ossifications on 

 the under side of the body called ventral or abdominal ribs, after 



Fig. 20. — Clavicles and interclavicle of 

 the Permocarboniferous of Mew Mexico. 



, a theromorph reptile from 



the eoracoids had become reduced and more slender. Whenever 

 it is present the coracoid articulates with it on each side in front. 

 In most lizards it remains as a cartilage throughout life. 



ANTERIOR EXTREMITY 



The upper arm bone, or humerus, like most other bones of the 

 extremities, has been greatly modified by the habits of the different 

 reptiles. In running and climbing reptiles it is always slender, 

 while in burrowing reptiles it is short and stout and much expanded 

 at the extremities, like the humerus of the mole among mammals. 

 And we shall also see how greatly modified it was among the 



