THE JURASSIC PERIOD. 245 



level with the surface from a considerable depth, may have 

 found a secure retreat from the assaults of powerful enemies ; 

 while the length and flexibility of its neck may have compen- 

 sated for the want of strength in its jaws, and its incapacity 

 for swift motion through the water." 



About twenty species of Plesiosaurns are known, ranging 

 from the Lias to the Chalk, and specimens have been found 

 indicating a length of from eighteen to twenty feet. The 

 nearly related " Pliosaurs" however, with their huge heads 

 and short necks, must have occasionally reached a length of at 

 least forty feet the skull in some species being eight, and the 

 paddles six or seven feet long, whilst the teeth are a foot in 

 length. 



Another extraordinary group of Jurassic Reptiles is that nf 

 the " Winged Lizards " or Pterosaur ia. These are often spoken 

 of collectively as " Pterodactyles," from Pterodadylus, the 

 type-genus of the group. As now restricted, however, the 

 genus Pterodactyhis is more Cretaceous than Jurassic, and it is 

 associated in the Oolitic rocks with the closely allied genera 

 Dimorphodon and Rhamphorhynchus. In all three of these 

 genera we have the same general structural organisation, in- 

 volving a marvellous combination of characters, which we are in 

 the habit of regarding as peculiar to Birds on the one hand, to 

 Reptiles on another hand, and to the Flying Mammals or 

 Bats in a third direction. The " Pterosaurs " are " Flying " 

 Reptiles, in the true sense of the term, since they were indu- 

 bitably possessed of the power of active locomotion in the air, 

 after the manner of Birds. The so-called " Flying " Reptiles 

 of the present day, such as the little Draco volans of the East 

 Indies and Indian Archipelago, possess, on the other hand, no 

 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 

 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, 



