264 REPORT OF THE AMERICAN COMMITTEE. 
tain uplifts, forming distinct hog-backs. The thickness is from 
three to four hundred feet. 
The 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 Avhich gives it character is a crocodilian reptile, with flat ar- 
ticular vertebral faces, provisionally referred to the genus Hypo- 
sauras. 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 Niobrara. — 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, vertebrate 
and invertebrate. Characterized as follows : 
Present: Pisces Isospondyli Saurodontidse, and Actinochiri, 
Hemibranchi Dercetidse; Reptilia Sauropterygia with long 
necks ; Pythonomorpha, except Mosasaurus ; Testudinata Pro- 
tostegidse; Pterosauria Pteranodontidse ; Aves Odontornithes. 
Absent : Reptilia Crocodilia Procosla ; Pythonomorpha, Mosa- 
saurus. 
Pierre. — Dark carbonaceous shales and clays, and dark-col- 
ored marls, which lie conformably on the Niobrara beds both on 
the flanks of the Rocky Mountains and on the northern parts of 
the Plains. Thickness (King), two hundred and fifty to three 
hundred feet. Represented in the East, according to Meek and 
Whitfield, by the lower green-sand marl of New Jersey, Dela- 
ware, etc. Invertebrate fossils very numerous ; vertebrates less 
numerous in the interior basin, more so on the Atlantic slope. 
The predominant genera in the two regions are Mosasaurus and 
Elasmosaurus, the latter occurring also in the Niobrara. The 
distinctness of this horizon from the latter on grounds of verte- 
brate paleontology depends chiefly on the fauna of the Eastern 
beds. The distinctions are : 
* See page 469 American Naturalist, May, 1887. 
