MESozoic. 263 
divisions of the Jurassic. Hence it has not been possible to 
refer the Rocky Mountain beds to any of the latter, and Marsh 
has therefore designated them as the Atlantosaurus beds. 
A series of deposits lies between the Triassic and Cretaceous 
formations in the middle Atlantic States, which have been sup- 
posed from the paleobotany to be of Jurassic age by Tyson. 
What division, if any, of the European series they represent has 
not yet been ascertained, but they are regarded by Mr. McGee 
as belonging at the summit of the system. He names them the 
Potomac formation in an unpublished memoir. 
CRETACIC SYSTEM. 
Characteristics. — Presence of Saurodont and Actinochirous 
fishes ; of Reptilia Eusuchia, Testudinata Protostegidse Pro- 
pleuridae, and Adocidse; of Aves Odontornithes. Absence of 
Pisces Ginglyraodi and Halecomorphi ; of Reptilia Choristdera, 
Dinosauria Opisthocoela. 
This formation has great extent and thickness in North Amer- 
ica, and it displays a number of divisions, which diiEFer both 
lithologically and faunally. These are the 
Foxhills, 
Pierre, 
Niobrara, 
Benton, 
Dakota, 
Comanche. 
The Comanche has been recently named and described by 
Hill. It consists principally of limestones of varying character 
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 given. 
The 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- 
