The Pythonomorpha are undoubtedly allied to the Sauria, but they are certainly not their ancestors, since typical Autosauri occur in the Lower Chalk; nor are the Snakes their descendants, in spite of many convergent resemblances. We consider them to be the marine collateral branch of the Sauria, which rapidly developed highly specialised, often very large forms, restricted to the Cretaceous epoch, with a wide, cosmopolitan distribution.
Order I. DOLICHOSAURI.
This older group is characterised by the sutural symphysial connexion of the two mandibles and by the possession of two sacral vertebrae. The body is snake-like. Pleurodont. Dolichosaurus longicollis of the Lower Chalk of Kent and Sussex; total length about 3 feet, with about seventeen cervical vertebrae and pleurodont teeth. Acteosaurus of Istria; anterior extremities distinctly shorter than the posterior pair; tail long. Vertebrae, like those of Dolichosaurus, with zygosphenes. Plioplatecarpus of the Upper Chalk of Holland has a slender interclavicle; the vertebrae are without zygosphenes, but those of the cervical region possess a downwardly directed long hypapophysial process with a separately ossified epiphysis.
Order II. MOSASAURI.
The two halves of the lower jaw are connected by ligament and are therefore movable as in Snakes. There are no sacral vertebrae, the pelvis having lost its connexion with the vertebral column. The formation of the limbs into paddles is more pronounced than in the Dolichosauri.
Mosasaurus, the chief genus, so called from Mosa, the Latin name of the river Maas, with several species from the Upper Cretaceous strata of the Netherlands, England, and North America. M. camperi, from Belgium, with a skull about 4 feet in length, armed with many large, curved, acrodont teeth. The vertebral column consists of about one hundred caudal and thirty-four precaudal vertebrae, of which seven are cervical, without zygosphenes. The total length of the type-specimen is estimated at 25 feet.
Platecarpus of North America and New Zealand, and various other North American genera, also contained species of large size.
Liodon.–Premaxilla without teeth, the others nearly smooth instead of being ridged. With a very wide distribution in the Chalk of Europe, North America, and New Zealand. L. haumuriensis of New Zealand seems to have been the giant amongst these monstrous marine creatures; its total length has been computed from imperfect fragments at 100 feet.
Clidastes, of the Upper Cretaceous of North America and Europe, although not so massive, comprises the most elongated forms. The cervical vertebrae possess long median hypapophyses with separate epiphyses; most of the vertebrae are much elongated and have well-developed zygosphenes. C. tortor had a skull nearly two feet and a half long.