270 THE OSTEOLOGY OF THE REPTn.ES 



vertebrae, slender cylindrical body of twenty-six or twenty-seven 

 vertebrae, two sacrals, and a long flattened tail. Zygosphenes pres- 

 ent. Legs relatively small, the front ones smaller than the hind. 

 Pleurodont. 



The doHchosaurs, with their greatly elongated neck and body, 

 have been thought by some to be ancestrally related to the snakes 

 but this is very doubtful, since their flattened tail shows a distinct 

 adaptation to water life and it is improbable that the snakes ever 

 passed through an aquatic stage in their evolution. Aside from the 

 Proganosauria, they are the only known swimming reptiles with 

 both neck and tail elongated. Just what habits were subserved by 

 this structure is a problem. Because of the snake-like sinuosity of 

 the neck, body, and tail, the small legs must have been of no pro- 

 peUing, and but little other, use in the water. Pleurosaurus, an allied 

 reptile of similar form, has a short neck. In all probability the doli- 

 chosaurs were a side branch from the common varanoid ancestral 

 stock of the aigialosaurs and mosasaurs, but not directly ancestral to 

 any later forms. 



Lower Cretaceous (Neocomion). Acteosaurus Meyer, Adriosaurns 

 Seeley, Pontosaurus Kramberger, Europe (Dalmatia). 



Upper Cretaceous. Dolichosaurus Owen, England. 



Family Aigialosauridae. Subaquatic lizards from three to six 

 feet in length. Skull large, mosasauroid. Neck of seven vertebrae; 

 body of twenty-one vertebrae; tail long, flattened. Two sacrals. 

 Legs of nearly equal size, the propodials somewhat shortened. 

 Feet not hyperphalangic, probably webbed. 



The skull of the aigialosaurs is almost identical in structure with 

 that of the mosasaurs, including the remarkable joint in the man- 

 dible between the angular and splenial, and their ligamentous union 

 in front. The neck is shortened, the body elongated, with the same 

 number of vertebrae found in some mosasaurs. The limbs, however, 

 were terrestrial, with only slight aquatic adaptations. Doubtless the 

 reptiles were amphibious in habit, frequenting the shallow waters 

 for food. 



Lower Cretaceous (Neocomion). Aigialosaunis Kramberger, Car- 

 sosaurus Kornhuber, Opetiosaurus Kornhuber, ? Mesoleptos CornaUa, 

 Europe (Dalmatia). 



