DIVISIONS OF EEPTILIA. 



251 



Secondary period of geology. The most familiar examples 

 are the so-called Pterodactyles, and the distinguishing charac- 

 ters of the order have reference to the fact that they were all 

 adapted for an aerial life. They present, in fact, an extraor- 

 dinary combination of the characters of birds and reptiles, and 

 they make also some approach to the Mammalian order of the 

 Bats. In the presence of teeth in distinct sockets, and, as we 

 shall see hereafter, in the structure of the fore-limbs, the 

 Pterodactyles differ altogether from all known birds; and 

 there can be little doubt as to their being genuine Reptiles. 

 The only living Reptile which has any power of sustaining 

 itself in the air is the little Draco volans, which has been pre- 

 viously mentioned. In this case, however, the animal has no 

 power of true flight, but is simply enabled to take extensive 

 leaps by means of a membranous expansion on each side of 

 the body. In the Bats, again, the power of genuine flight is 

 present ; and this is given by means of a leathery membrane 

 which is supported chiefly by certain of the fingers — which 

 are greatly lengthened — and is attached to the sides of the 

 body and hind-limbs. 



In the Pterodactyles the power of true flight was present, 

 and this was also conditioned by means of a leathery expand- 

 ed membrane, attached to the hind-limbs, the sides of the 

 body, and the fore-limbs. In this case, however, the chief 

 support of the flying membrane was derived from the outer- 

 most finger of the fore-limb, which was enormously elongated 

 (Fig. 137). That the Pterodactyles passed their existence 



Fig. 12T.—Pterodac^lm bremrostris, the skeleton and th6 animal restored. 



chiefly in the air, and did not simply leap from tree to tree, is 

 shown by two characters in which they agree with the flying 



