VERTEBRATA: REITILIA. 565 



little question as to their being genuine Reptiles. The only 

 Reptiles, however, now existing, which possess any power of 

 sustaining themselves in the air, are the little Dragons (Draco], 

 but these can only take extended leaps from tree to tree, and 

 cannot be said to have any power of flight properly so called. 

 That the Pterodactyles, on the other hand, possessed the 

 power of genuine flight, is shown by the presence of a median 

 keel upon the sternum, proving the existence of unusually de- 

 veloped 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 existence of air-cavities in" the bones, giving the animal 

 the necessary 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 Mam- 

 malian 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. The diffi- 

 culty 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 Chalk inclusive, the Lithographic Slate 

 of Solenhofen (Upper Oolite) being particularly rich in their 

 remains. Most of them appear to 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 

 with teeth to their extremities, all the teeth being long and 

 slender. 



In Dimorphodon, the anterior teeth are large and pointed, 

 the posterior teeth small and lancet-shaped. 



In Ramphorhynchus, the anterior portion of both jaws is 

 edentulous, and may have formed a horny beak, but teeth 

 are present in the hinder portion of the jaws. 



In Pteranodon, lastly, the jaws are completely edentulous, 

 and were probably ensheathed in horn. This genus, along 

 with some small forms, includes the largest known members 

 of the order. 



ORDER IX. DINOSAURIA, or DEINOSAURIA. The last order 

 of extinct Reptiles is that of the Dinosauria, comprising a 

 group of very remarkable Reptiles, which are in some respects 



