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Marine reptiles from eight to about fifty feet in length, with paddle-like, hyperphalangic limbs. Skull moderately broad to very slender. Nares small, situated remote from the extremity and near the orbits. Orbits with sclerotic plates. No distinct nasals. Internal nares small, situated in front of the external. A pair of posterior interpterygoidal openings divided by the parasphenoid always present; other openings variable on the palate. The squamosals meet in the middle line posteriorly. Coracoids very large, contiguous in midline; clavicles and interclavicle small, sometimes vestigial. Ilium rod-like, articulating below with ischium only, above with a well-developed sacrum of three or four vertebrae.

An extensive and long-lived group of purely marine reptiles, widely distributed over the earth; as a whole clearly defined, but with many minor modifications. The neck was extremely variable in length, with from thirteen to seventy-six cervical vertebrae. The body was broad, though not nearly so broad as represented in most modern restorations. The most perfect specimen known—and the author has seen most of them in the collections of the world—is that of Thaumatosaurus victor, in the Stuttgart museum, of which a figure copied from a photograph is reproduced here. The body, it is seen, is broadly oval, but not flat, protected below by the extraordinary developments of the pectoral and pelvic girdles and intervening parasternal ribs. Their phylogenetic relationships with the Nothosauria are incontestable, though the closed palate of the latter indicates that no known form could have been actually ancestral to them.

. Skull moderately long. From thirty-five to [seventy-six] cervical vertebrae, the cervical ribs double-headed. Scapulae not contiguous in the middle; no interclavicular foramen; epipodials much longer than broad, no accessory epipodials. Coracoids contiguous throughout.

Jurassic. Plesiosaurus Conybeare, Thaumatosaurus Meyer, Europe.

. Skull long, neck short, composed of about nineteen vertebrae. Cervical ribs double-headed; five pectoral and about twenty dorsal vertebrae. Premaxillae continuous to parietals in middle. Scapulae closely approximate in midline; coracoids