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. 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 Ptero