Page:The Osteology of the Reptiles.pdf/288

270 vertebrae, slender cylindrical body of twenty-six or twenty-seven vertebrae, two sacrals, and a long flattened tail. Zygosphenes present. Legs relatively small, the front ones smaller than the hind. Pleurodont.

The dolichosaurs, 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 propelling, and but little other, use in the water. Pleurosaurus, an allied reptile of similar form, has a short neck. In all probability the dolichosaurs 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, Adriosaurus Seeley, Pontosaurus Kramberger, Europe (Dalmatia).

Upper Cretaceous. Dolichosaurus Owen, England.

. 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 mandible 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). Aigialosaurus Kramberger, Carsosaurus Kornhuber, Opetiosaurus Kornhuber, ? Mesoleptos Cornalia, Europe (Dalmatia).