DIVISIONS OF REPTILIA. 275 
confined to the Secondary period of geology. The most fami- 
liar examples are the so-called Pterodactyles, and the distin- 
guishing characters of the order have reference to the fact that 
they were all adapted for an aerial life. They present, in fact, 
an extraordinary 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 previously 
mentioned. In this case, however, the animal has no power of 
true flight, but is simply enabled to take extensive leaps by 
Fig. 150.—Prerodactylus brevirostris, the skeleton and the animal restored. 
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 expanded 
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 outermost finger of 
the fore-limb, which was enormously elongated (fig. 150). That 
the Pterodactyles passed their existence chiefly in the air, and 
did not simply leap from tree to tree, is shown by two characters 
