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 (Ophidia) have as yet been detected in the Cretaceous rocks; and this order does not appear to have come into existence till the Tertiary period.  Lastly, true Crocodiles are known to have existed in considerable numbers in the Cretaceous period.  The oldest of these occur in the fresh-water deposit of the Wealden; and they differ from the existing forms of the group in the fact that the bodies of the vertebrae, like those of the Jurassic Crocodiles, are bi-concave, or hollowed out at both ends.  In the Greensand of North America, however, occur the remains of Crocodiles which agree with all the living species in having the bodies of the vertebrae in the region of the back hollowed out in front and convex behind.