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Large marine lizards with more or less elongated head, shortened neck, elongated body, a long, flattened tail with a more or less subterminal dilatation, and paddle-like extremities. From six to about forty feet in length. Temporal and postorbital arches complete, the tabular with a long process wedged in between paroccipital and proötic. Parietal and frontal unpaired; a parietal foramen. Palate with large openings. Teeth with osseous base inserted in shallow pits in premaxillae, maxillae, dentaries, and pterygoids. Nasals and premaxillae fused into a single bone. A true joint between angular and splenial; rami of mandibles united by ligaments. Vertebrae procoelous. Sclerotic plates present sometimes with zygosphenes. Seven cervicals. No clavicles; sometimes a slender interclavicle. A calcified sternum. No sacrum. Legs paddle-like, short, webbed, without claws, hyperphalangic, pentadactylate.

The mosasaurs are a group of large marine lizards, of world-wide distribution during Upper Cretaceous times. In all probability they were descended from subaquatic lizards like the aigialosaurs in late Lower Cretaceous times, differing from them chiefly in the loss of the sacrum and the adaptation of their limbs to purely aquatic uses.

Three types of mosasaurs are recognized: the surface-swimming type with elongated trunk composed of as many as thirty-five dorsals, the tail with a pronounced subterminal dilatation, zygosphenes, a well-ossified carpus, and only slight hyperphalangy, of which Mosasaurus and Clidastes are types; a deeper-sea type with proportionally shorter neck, less elongated trunk with but twenty-two vertebrae, a more uniformly flattened tail, less well-ossified carpus and tarsus, and greater hyperphalangy, with Platecarpus as a type; a diving type, with more elongated head, heavy cartilaginous protections for the ears, a relatively short neck, body with but twenty-two vertebrae, a longer and much flattened tail, the almost entirely cartilaginous mesopodials and highly developed hyperphalangy, and greater size, of which Tylosaurus is the best-known type. And these three groups have been, perhaps rightly, recognized as distinct families. The mosasaurs were clothed with small Varanus-like scales, of which impressions have been often found. The bones, especially of the deep-diving forms, were soft, doubtless impregnated in life with fat.