272 THE OSTEOLOGY OF THE REPTILES 



Tribe Pythonomorpha (Mosasauria) 



Large marine lizards with more or less elongated head, shortened 

 neck, elongated body, a long, flattened tail with a more or less sub- 

 terminal 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 

 prootic. 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 dor- 

 sals, 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 propor- 

 tionally 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 pro- 

 tections 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 fam- 

 ilies. The mosasaurs were clothed with small Fara«w5-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. 



