624 REPT1LIA. 



girdle, but they differ markedly in their vertebral column, pelvis, and 

 organ of flight. An expansion of the skin seems to have been stretched 

 on the much elongated outermost finger, and to have extended back- 

 wards to the hind -legs and the tail. The long bones are hollow. The 

 sternum is keeled, and teeth are often present on the margin of both 

 jaws. There is both a superior and an inferior temporal arcade. The 

 quadrate is fixed. Some were no larger than sparrows, but others 

 the giants with which the race ended had in some cases a spread of 

 wing of nearly 20 ft. It is a question how far the resemblances of these 

 forms to Birds are a consequence of similar habits, and how far they 

 can be regarded as indicating true affinities. 



Examples. Pterodactyhts, Rhamphorhynchus^ Pteranodon. 



RELATIONSHIPS. 



Reptiles, in their widest sense, form a central assemblage 

 among Vertebrates. As we have noted above, 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. 



