THE JURASSIC PERIOD. 248 
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 Plesiosaurus 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 skullin 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 of 
the “ Winged Lizards” or Prerosauria. ‘These are often spoken 
of collectively as ‘ Pterodactyles,” from /¢erodactylus, the 
type-genus of the group. As now restricted, however, the 
genus Prerodactylus is more Cretaceous than Jurassic, and it is 
associated in the Oolitic rocks with the closely allied genera 
Dimorphedon 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 hmbs, 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, 
