us_, or
absolutely belonging to it, have been discovered in the Cretaceous
deposits of North America, and have been described by Professors
Cope and Marsh. All the known forms of this group appear to have
been of large size--one of them, _Mosasaurus princeps_, attaining
the length of seventy-five or eighty feet, and thus rivalling
the largest of existing Whales in its dimensions. The teeth in
the "Mosasauroids" are long, pointed, and slightly curved; and
instead of being sunk in distinct sockets, they are firmly
amalgamated with the jaws, as in modern Lizards. The palate also
carried teeth, and the lower jaw was so constructed as to allow
of the mouth being opened to an immense width, somewhat as in the
living Serpents. The body was long and snake-like, with a very
long tail, which is laterally compressed, and must have served as
a powerful swimming-apparatus. In addition to this, both pairs
of limbs have the bones connecting them with the trunk greatly
shortened; whilst the digits were enclosed in the integuments,
and constituted paddles, closely resembling in structure the
"flippers" of Whales and Dolphins. The neck is sometimes moderately
long, but oftener very short, as the great size and weight of
the head would have led one to anticipate. Bony plates seem in
some species to have formed an at any rate partial covering to
the skin; but it is not certain that these integumentary appendages
were present in all. Upon the whole, there can be no doubt but
that the Mosasauroid Reptiles--the true "Sea-serpents" of the
Cretaceous period--were essentially aquatic in their habits,
frequenting the sea, and only occasionally coming to the land.

[Illustration: Fig. 210.--Skull of _Mosasaurus Camperi_, greatly
reduced. Maestricht Chalk.]

The "Mosasauroids" have generally been regarded as a greatly
modified group of the Lizards (_Lacertilia_). Whether this reference
be correct or not--and recent investigations render it dubious--the
Cretaceous rocks have yielded the remains of small Lizards not widely
removed from existing forms. The recent order of the _Chelonians_
is also represented in the Cretaceous rocks, by forms closely
resembling living types. Thus the fresh-water deposits of the
Wealden have yielded examples of the "Terrapins" or "Mud-Turtles"
(_Emys_); and the marine Cretaceous strata have been found to
contain the remains of various species of Turtles, one of which
is here figured (fig. 211). No true Serpents (_Op