In America, members of the group have been discovered in the Cretaceous deposits of New Jersey, Alabama, North Carolina, the upper Missouri region, Nebraska, Kansas and New Mexico. Probably nineteen-twentieths of all the known specimens, however, have been obtained in western Kansas. The material now in the University Museum, all from Kansas, comprises several hundred specimens of these animals, including, probably, the best ones known. It is upon this material that the following preliminary studies are chiefly based. The genus _Clidastes_, as first described by Cope, was based upon two dorsal vertebrae of _C. iguanavus_, the type species, from New Jersey. Shortly afterward, however, he gave a full and careful generic description, as derived from an unusually good specimen of an allied species, _C. propython_, from Alabama. Only a little later, Marsh described a genus, which he called _Edestosaurus_, from Kansas, but without giving any real, distinctive differences from _Clidastes_, following the very reprehensible practice of naming supposed new forms in the hopes that future distinctive characters might be found. The genus _Edestosaurus_ has been rejected by nearly all save the authors of the American text-books in Geology. It seems hardly necessary to point out the identity. The only distinctive character the author gave for his genus was the insertion of the pterygoid teeth, and even this character he modified later—“Palatine (sic) teeth more or less pleurodont.”[6] [6] Amer. Journ. Sci. iii, June 1872. This character, even were it real, is of very slight value; indeed it cannot be used to distinguish the species even. _Clidastes_ is, without doubt, one of the most highly specialized genera in the group, and, what is very interesting, is one of the latest. It occurs in Kansas in the uppermost part of the Niobrara beds, in the horizon so markedly characterized by the toothed birds. Both _Platecarpus_ and _Liodon_ occur, though in diminished numbers, almost to the very lowest portion, but _Clidastes_ has never been found except towards the top. From measurements made the past season, the thickness of the beds in which these saurians occur cannot be less than six hundred feet. The following species have been found in Kansas: none of them are known to occur elsewhere. MOSASAURIDAE. _Mosasauridae_ Conybeare, in Cuvier, Ossem. Foss., 2nd ed., p. 338, 1824. _Clidastidae_ Cope, Extinct Batr. Rept. and Aves of N. Amer., Trans. Amer. Phil. Soc. xiv, p. 50, 1870. _Edestosauridae_ Marsh, Amer. Journ. Sci. xxi, p. 59, July 1878. CLIDASTES. ? _Geosaurus_ Cuvier, Ossem. Foss. 2nd ed., 328, 1824, (_fide_ Lydekker.) _Clidastes_ Cope, Proc. Acad. Nat. Sci. Phil. 1868, p. 233; Ext. Batr. etc., p. 21, 1870. _Edestosaurus_ Marsh, Amer. Journ. Sci. i, p. 417, June, 1871. =C. cineriarum.=