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Crawling or swimming reptiles from three to seven feet in length, of exclusively Triassic age. Skull depressed, more or less elongate, the orbits situated far forward, looking upward. Nares about midway between the orbits and extremity. Lacrimals possibly absent. Palate without openings, except the large internal nares, the vomers and pterygoids meeting in the middle line throughout. From twenty to twenty-five cervical, twenty-five to thirty dorsal, two to five sacral, vertebrae, and a moderately long tail. Clavicles stout, the interclavicle vestigial. The elongated coracoids meet in the middle line. Epipodials much shorter than propodials. Phalangeal formula primitive, or with the loss of one phalanx in the fourth finger. Digits probably webbed in life.

The Nothosauria were all aquatic in habit, but not exclusively so like the plesiosaurs, the feet still retaining terrestrial characters, with but minor aquatic adaptations. The parasternals, like those of the Plesiosauria, are very stout, apparently also an aquatic adaptation. The body was never slender, though less broad than that of the plesiosaurs, and it is not probable that they were rapid swimmers. They doubtless lived in the shallow waters, as do the crocodiles, coming frequently to land, and subsisted chiefly upon fishes and invertebrates, for the capture of which their slender, curved teeth were well fitted. A peculiar parallel adaptation to that of the contemporary aquatic Labyrinthodontia is seen in the forward position of the eyes in the flat skull, and also in the unusually stout clavicular girdle of both.

Several families have been proposed, based upon minor characters of the skull chiefly. For the present they may all be placed in a single family, the Nothosauridae.

. Upper and Middle Trias. Anarosaurus Dames, Cymatosaurus Fritsch, Dactylosaurus Gürich, Doliovertebra Huene, Lamprosaurus Meyer, Lariosaurus Curioni, Microleptosaurus Scuphos, Neusticosaurus Seeley, Nothosaurus Münster, Parthanosaurus Scuphos, Pistosaurus Meyer, Proneusticosaurus Volz, Simosaurus Meyer.