Mosasaurus, a gigantic fossil reptile, so . named by Conybeare from its having been first found on the banks of the river Maas, near Maestricht in Holland, in the upper cretaceous formations of that district. It was referred to the orders of cetaceans and crocodilians, but A. Camper and Cuvier showed from the teeth and the skeleton that its true place was between the monitors and the iguanian lizards. The bones of the head are like those of monitors; the teeth of the jaws are compressed, sharp-edged, supported on a socket in a shallow furrow, without true roots; there are teeth also on the pterygoid hones, as in the iguanians; there seem to have been 28 teeth in each jaw, with broad base and sliflrhtly curved. The head is elongated, and the mouth wide; the vertebras concavo-convex, 34 from head to tail, the latter having nearly 100; as the articular processes are absent from the middle of the back, it has been inferred that the body possessed greater flexibility than that of the crocodiles; the tail is compressed laterally, and has strong V-shaped bones below, indicating its use as a powerful swimming organ; the ribs have a single head; the humerus short and thick as in the ichthyosaurus, and the limbs probably flattened into fins as in enaliosaurians.
It must have been a very active marine carnivorous animal. The best known species, M. Camperi (Conyb.), had a head 4 ft. long, while that of the largest living monitor is only 5 in., and the length of the animal must have been 25 ft. After the capture of Maestricht by the French (1794), these bones were sent to Paris. (See Cuvier's Ossemens fossiles.) This genus was abundant on the coast of New Jersey in the cretaceous epoch, and ten species have been described in the United States. Some of them attained a length of more than 50 ft. Their movements must have been rapid, by lateral undulations in the water, and serpent-like on hind; according to Prof. Cope, thev were probably able to coil themselves like snakes; they were veritable sea serpents of the cretaceous seas. Clidastes and macroxaurus were smaller and more slender allied genera. The genusgeotaurm (Cuv.), found in the calcareous schists of Monheim, Bavaria, came nearer the crocodiles in the pelvis and thigh bones. - See "Proceedings of the Academy of Natural Sciences," pp. 91, 92 (Philadelphia, 1859).