Another remarkable group of Reptiles, exclusively confined to the Cretaceous series, is that of the _Mosasauroids_, so called from the type-genus _Mosasaurus_. The first species of _Mosasaurus_ known to science was the _M. Camperi_ (fig. 210), the skull of which--six feet in length--was discovered in 1780 in the Maestricht Chalk at Maestricht. As this town stands on the river Meuse, the name of _Mosasaurus_ ("Lizard of the Meuse") was applied to this immense Reptile. Of late years the remains of a large number of Reptiles more or less closely related to _Mosasaurus_, 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.]