448 Mesozoic and Cenozoic of North America. [May ` 
which contain numerous marine invertebrata, which have been 
determined by White to represent a horizon of the Cretacic 
lower than the Dakota, and corresponding with some member of 
the Lower Cretacic of Europe. No vertebrates known. The 
formation is seen between the east and west Cross-Timbers of 
Texas, and the thickness is not givén. 
Tue Dakota? has not yet produced vertebrate remains, but 
abounds in plants which have, according to Lesquereux,”* the 
character of those of the Turonian or Lower Chalk of Europe, 
with an admixture of Miocene and recent types. Its beds con- 
sist of generally hard sandstone and conglomerate, and they 
occur almost everywhere along the flanks of the Rocky Moun- 
tain uplifts, forming distinct hog-backs. The thickness is from 
three to four hundred feet. 
Tue Benton.—These beds consist of dark-colored clays, more 
or less shaly, and have a thickness of from two hundred to four 
hundred and fifty feet. They contain vertebrate fossils, mostly 
fishes in poor preservation. The only vertebrate type observed 
in it which gives it character is a crocodilian reptile, with flat ar- 
ticular vertebral faces, provisionally referred to the genus Hypo- 
saurus.*5 The Benton formation is widely distributed, usually 
» present where the Dakota occurs, and lying conformably on it, 
and from its soft material, forming valleys. 
THE Niosprara.—Composed of harder and softer argillaceous 
limestones and chalky marls, varying from one hundred to two 
hundred feet in thickness.” The Niobrara is present with the 
-Dakota and Benton on the flanks of the Rocky Mountains, but 
has also a wide extent east and southeast of them, forming a 
large part of The Plains, and other large tracts in Texas.* It 
probably occurs in the valley of the Red River of the. North. 
It is a deep-water formation, and is very rich in fossils, verte- 
brate and invertebrate. Characterized as follows: 
Present: Pisces Isospondyli Saurodontide, and Actinochiri, 
Hemibranchi Dercetide;*7 Reptilia Sauropterygia with long 
nee Pythonomorpha, except Mosasaurus; Testudinata Pro- 
tostegidz ; Pterosauria Pteranodontide ; ** Aes Odontornithes. 
—— —— Crocodilia Proccela; Ryihenomore ait osasau- 
mete ig, ORE + 1...1 oe | 
x lays, and dark-colored 
; "Seepage 469 this mumber Awenicas NATURALIST. 
