THE JURASSIC PERIOD. 253 



power of genuine flight, being merely able to sustain themselves 

 in the air through the extensive leaps which they take from tree 

 to tree, the wing-like expansions of the skin simply exercising 

 the mechanical function of a parachute. The apparatus of flight 

 in the " Pterosaurs " is of the most remarkable character, and 

 most resembles the " wing " of a Bat, though very different in 

 some important particulars. The " wing " of the Pterosaurs is 

 like that of Bats, namely, in consisting of a thin leathery expan- 

 sion of the skin which is attached to the sides of the body, and 

 stretches between the fore and hind limbs, being mainly sup- 

 ported by an enormous elongation of certain of the digits of 



Fig. 11%,Pterodactylua crasairostris. From the Lithographic Slates of Solenhofen 

 (Middle Oolite. ) The figure is " restored," and it seems certain that the restoration 

 is incorrect in the comparatively unimportant particular, that the hand should consist 

 of no more than four fingers, three short and one long, instead of five, as represented. 



the hand. In the Bats, it is the four outer fingers which are 

 thus lengthened out; but in the Pterosaurs, the wing-membrane 

 is borne by a single immensely-extended finger (fig. 178). 

 No trace of the actual wing-membrane itself has, of course, 

 been found fossilized ; but we could determine that the " Ptero- 

 dactyles " possessed the power of flight, quite apart from the ex- 

 traordinary conformation of the hand. The proofs of this are to 

 be found partly in the fact that the breast-bone was furnished 

 with an elevated ridge or keel, serving for the attachment of 

 the great muscles of flight, and still more in the fact that the 



