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. 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 expansion of the skin which is attached to the sides of the body, and stretches between the fore and hind limbs, being mainly supported 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, been found fossilised; but we could determine that the "Pterodactyles" possessed the power of flight, quite apart from the extraordinary conformation of the hand. The proofs of this are to be found partly in the fact