31 



they were of an elongate snake-like form, which the two pairs of short 

 paddles did not materially modify. These limbs were situated in the 

 anterior half of the length, the resulting form being like that of some of 

 the snake-like terrestrial lizards at present inhabiting various countries, 

 especially of the southern hemisphere. Although I do not possess any 

 specimen with complete vertebral column, an approximate idea of its 

 length may be gained by comparison of parts which are more or less 

 complete in different species. Thus in Clidastes propython the cervicals 

 and dorsals number about 33 ; in a Liodon latispinus the lumbars num- 

 ber 7, and a series of caudals with diapophyses 32, with but little diminu- 

 tion in size, the last with stout bijt reduced diapophysis. Caudals without 

 diapophysis in a species of Platecarpus number 27, and there were at 

 least as m any, probably a larger number, beyond these. 



The well-distinguished genera of the order known from l!^orth Ameri- 

 can strata are the following : 



I. Cervical hypopophyses, separate, articulating. 



A zygosphenal articulation. 



Chevron bones co-ossified with centra Clidastes 



Chevron bones free .- Sironectes 



No zygosphenal articulation. 



Teeth subcylindric facetted ; chevron bones free . . . Platecarpus 

 Teeth mostly compressed, cutting; humerus with narrow ex- 

 tremities; chevron bones free Liodon 



Teeth subcylindric facetted ; chevron bones co-ossifled 



Mosasaurus 



II. Cervical hypophophyses continuous and entire. 



No zygosi)lien Baptosaurus 



CLIDASTES, Cope. 



Proceedings Academy Philadelphia, 1368, p. 233 ; Transactions American Philosophical 

 Society, 1870, p. 211 ; Edestosaurm, Marsh, American Journal of Science and Arts, 

 1871, June; Cope, Proceedings American Philosophic Society, 1871, December. 



There are specific differences in the form of the palatine bones in this 

 genus from the more transverse or expanded type of the C. propython 

 to that of the G.planifrons^ where they are narrowed posteriorly in some 

 degree, to the G. tortor, where they are vertically placed in the posterior 

 half. Clidastes is nearly added to Platecarpus, with which Sironectes 

 associates it as an intermediate genus. The number of species already 

 known is considerable, and the genus is divided into sections for con- 

 venience of reference. 



A. Centra of dorsal vertebrae depressed. 



a. Frontal bones without median keel. 



Clidastes planipeonSj Sp. nov. 



A large species represented by large portions of the cranium includ- 

 ing quadrate bone, by cervical and dorsal vertebrae and fragments of 

 other elements all belonging to one individual. They are well preserved 

 and have suffered but little from distortion. 



The frontal bone is especially massive, and is jilane on the superior 

 surface. The suj)erciliary borders are strongly concave, a feature 

 either little or not at all marked in other species known to me. It is 

 thickened, but the fossa of the postfrontal bone extends far toward the 

 front and middle on the inferior surface. Anterior to the prefrontal 



