C0PE.1 PALEONTOLOGY CRETACEOUS PERIOD AGE. 431 



CHAPTEE II. 



THE CRETACEOUS PERIOD. 



Section I. — on the mutual relations of the cretaceous and 



TERTIARY FORMATIONS OF THE WEST. 



The subject which it is proposed here briefly to discuss is one which 

 has excited considerable interest for several reasons. One of these is, 

 that there exists some discrepancy in the evidences as to the true age of 

 beds at the summit of the Cretaceous period and base of the Tertiary 

 in the Missouri and Eocky Mountain regions, and hence a difference of 

 opinion. Another is, that the question of continuity in topographical, 

 and hence of faunal and floral, relations, will be largely elucidated by a 

 proper determination of the beds in question, both geologically and 

 paleontologically. I have endeavored to attain some results in the 

 latter field in the department of Vertebrata, which are here presented, 

 with some stratigraphical observations made at localities either little or 

 not previously studied. 



Messrs. Meek and flayden have classified the vast thickness of the 

 Cretaceous system, recognizing five epochs as quite distinctly defined. 

 These are as follows : 



I. The Dakota group, (No. 1.)— The present list does not include any 

 species as discovered in this formation. Developed on the Missouri and 

 on the Eio Grande, New Mexico. 



II. The Benton group. — Seen on the Missouri Eiver by Hayden, and 

 stated by him to extend to the Smoky Hill Eiver, in Kansas, and to 

 Texas. I have determined only three species from it, namely : Sypo- 

 Sdurus vehhii, a crocodile ; Apsopelix sauriformis, a clupeoid ; and Pele- 

 corapis varians, a ctenoid fish. Other species of fishes occur in the same 

 formation in Kansas. 



III. The Niobrara group. — From the Missouri, Kansas, and Texas, 

 according to Hayden. Confirmatory of the last locality are remains of 

 Fythonomorpha from that State, discovered and sent to me by Mr. A. E. 

 Eoessler. I have also described a species of that order as common to 

 Eastern New Mexico and Western Kansas ; and Hayden and Lecoute 

 state that it appears north of the Arkansas in Southern Colorado. 

 Vertebrate remains are abundant in this formation, and it has furnished 

 a majority of those investigated by paleontologists. They are distributed 

 as follows, among the orders of Vertebrata ; 



Aves : 



Natatores 2 



(?) tSaururce 2 



Eeptilia : 



Dinosauria i 1 



Pterosauria 4 



Sauropterygia 3 



Tesiudinata 3 



PythonomorpJia : 27 



