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250 throughout. Two or three epipodials, as broad as long or broader. Ischia long. Large or very large.

Jurassic. Pliosaurus Owen, Peloneustes Lydekker, Europe.

. Very much like the following family, but the neck is shorter, with from thirty-two to forty-four vertebrae; and the coracoids are contiguous throughout. From two to four epipodials, all short. Cervical ribs single-headed. Skull short.

Jurassic. Cryptocleidus Seeley, Muraenosaurus Seeley, Tricleidus Andrews, Picrocleidus Andrews, Microcleidus Watson, Sthenarosaurus Watson, Europe.



. Head short, neck very long, with from more than fifty to seventy-six vertebrae; ribs single-headed. The scapulae meet in midline; no interclavicular foramen. Coracoids broadly separated on their posterior half. Ischia short. Two epipodials only, short.

Upper Cretaceous. Elasmosaurus Cope, Ogmodeirus Williston and Moodie, Leurospondylus Brown, North America.

. Skull very slender. Premaxillae articulating with parietals. Neck not longer than head, with from twenty-three to twenty-six vertebrae; ribs single-headed. The precoracoidal process separates the scapulae in the midline; an interclavicular foramen; coracoids contiguous throughout. Ischia elongate. Three or four epipodials, all short.

Upper Cretaceous. Polycotylus Cope, Trinacromerum Cragin, ? Piratosaurus Leidy, North America.