PTEROSAURIA. 375 



the sternum, proving the existence of unusually-developed 

 pectoral muscles ; by the articulation of the coracoid bones 

 with the top of the sternum, providing a fixed point or fulcrum 

 for the action of the pectoral muscles ; and, lastly, by the exist- 

 ence of air-cavities in the bones, giving the animal the neces- 

 sary degree of lightness. The apparatus, however, of flight was 

 not a " wing," as in Birds, but a flying-membrane, very similar 

 in its mode of action to the patagium of the Mammalian order 

 of the Bats. The patagium of the Bats, however, differs from 

 that of the Pterodactyles in being supported by the greatly- 

 elongated fingers, whereas in the latter it is only the outermost 

 finger which is thus lengthened out. 



Fig. 324. Pterodactylus crassirostris. From the Lithographic Slates of Solenhofen. 

 (Upper Oolite). 



The difficulty as to the position of the Pterosauria is evaded 

 by Mr Seeley by placing them in a distinct class, which he 

 terms Ornithosauria, and which he regards as most nearly 

 related to, but coequal with, the class Aves. 



The Pterosauria are exclusively Mesozoic, being found from 

 the Lower Lias to the Middle Chalk inclusive, the Lithographic 

 Slate of Solenhofen (Upper Oolite) being particularly rich in 

 their remains. Most of them appear tfi have attained no very 

 great size, but the remains of a species from the Cretaceous 

 Rocks have been considered to indicate an animal with more 

 than twenty feet expanse of wing, counting from tip to tip. 



In the genus Pterodactylus proper, the jaws are provided 



