6 Hall on the Strata and Organic Remains 
inferior calcareous strata, has received, in the Ohio reports, the — 
name of BLUE Limestone. The thickness of the deposit is ap- - 
parently less than in New York; the arenaceous matter has | 
nearly disappeared, while the argillaceous portion is much di- 
minished, and instead of shaly slate, is, for the most part, an . 
incoherent marl, or soft shale. Calcareous matter at the same 
time has greatly increased, so that from forming scarcely an 
important feature at the east, it is the predominating material. — 
We are now to inquire what changes, during the period of this — 
deposit, occurred in the organic beings inhabiting the ocean — 
bed. : 
In New York, shells of the Dimyaria, as Modiola, Cypri- - 
cardia, and Nucula, with Pterinea, are the prevailing forms ; ‘ 
while the Brachiopoda, with the exception of two or three spe- — 
cies, are rare. In Kentucky, Ohio and Indiana, the Brachiopo- — 
da, in the forms of Atrypa, Delthyris, Orthis and Strophomena : 
abound, to the almost entire exclusion of the predominating 
genera of New York. Itis true that the fossils characteristic — 
of the same strata in New York are found, but would by no - 
means be considered the typical fossils of the rock at the west. | 
Cyrtolites ornatus, Pterinea carinata, and one or two Modi- 
ola-like fossils, which are the reliable forms with us, are found — 
associated with a far greater proportion of Strophomena, Or- - 
this, Delthyris, Atrypa, being in fact the least prominent fos- 
sils of the group. ie 
The same character of strata, just noticed, holds good upon 
the Mississippi River, and the same folsils are found as in 
Strophomena are usually the only New York fossils of tl 
group at the west, which attract notice. - per 
. Corals and Crinoidea, are infinitely more abundant in this 
group throughout all western localities than in New York. 
These are probably dependent upon, and in some degree a 
source of, the great predominance of calcareous matter. - The 
‘Crustacea have greatly increased in numbers over those of the 
same group in New York, though the species there con 
