T. A. Conrad on American Fossiliferous Strata. 359 
Tertiary of Maryland. This group of extinct freshwater bi- 
valves seems to have been more uniform in character than 
those of the present day, since it is generally the same on both 
‘Sides of the Appalachian range, where now the mountains 
separate them into two very dissimilar groups, one plain and few 
in number, and the other numerous, with every variety of form. 
It is this uniform character, together with the extinction of 
the shells which lead to the conclusion of their Miocene age. 
Of the Pliocenes we have no trace along the Atlantic slope, 
and therefore the geological position must be either that of 
the Miocene, or of the Post Pliocene, in which latter formation 
the species either marine or fresh water are identi ith 
existing forms. The Ohio deposit containing extinct species is 
thus described by Dr. Hildreth. They are “ina bed of fine 
Micaceous and siliceous sand, the upper part mixed with blue 
clay.” This clay is identical in composition and color with 
that of the Delaware, containing the same amount of micaceous 
fragments, showing that both deposits were filled up with sim- 
ilar sediment, and at the same period. The fossils of Ohio 
are mineralogically the same as those of the Delaware and 
Potomac, ferruginous casts with portions of the shell re- 
Maining. The former are at an elevation of about 600 feet 
Crosswicks GROUP. 
The lower beds of the eastern Cretaceous are exposed in the 
.." Prof. Emmons found portions of Equus fraternus Leidy, in a Miocene marl 
pit, near Elizabeth, Bladen Co., N. C., and other specimens in the Miocene of 
