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.