neck must have impeded its progress through the water, presenting a strong contrast to the organisation which so admirably fits the Ichthyosaurus to cut through the waves.”  As its respiratory organs were such that it must of necessity have required to obtain air frequently, we may conclude “that it swam upon or near the surface, arching back its long neck like a swan, and occasionally darting it down at the fish which happened to float within its reach.  It may perhaps have lurked in shoal water along the coast, concealed amongst the sea-weed; and raising its nostrils to a 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 compensated 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 skull in some species being eight, and the paddles six or seven feet long, whilst the teeth are a foot in length.

[Illustration:  Fig. 178.—­Pterodactylus crassirostis.  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.]

Another extraordinary group of Jurassic Reptiles is that of the “Winged Lizards” or Pterosauria.  These are often spoken of collectively as “Pterodactyles,” from Pterodactylus, the type-genus of the group.  As now restricted, however, the genus Pterodactylus 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, involving 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 indubitably 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.