Extinction of Species. — Mc Creery. 101 
added to every year, yet not a single one of these species is 
living to-day. 
Between 250 and 300 fossil forms have been recognized in 
the 800 feet of limestone and shales known under the general 
name of the "Cincinnati group" and by following "the testi- 
mony of the rocks" we may learn something of the life history 
of at least a few of this long array of species. 
In the Trenton limestone, the lowest rock exposed in Ohio 
and the formation in which natural gas is obtained, there is 
found a fossil brachiopod shell, Orthis biforata. It occurs 
very sparingly in the fifty feet exposed at Pt. Pleasant, 
Clermont county, and is seldom or never met with in the lower 
portion of the next higher deposit, the Cincinnati beds. But 
in the upper portion; at the hight of 300 feet above low 
water mark of the Ohio river, this fossil becomes quite com- 
mon. It is, however, a smaller shell and so different from 
the typical form that it is classed as a variety of 0. biforata 
under the name of var : dentata. About 50 feet higher or 350 
feet above low water, it has reached the full size of the typical 
0. biforata but does not show all its peculiarities and is known 
to science as var : lynx. At the hight of 425 feet it assumes 
all the characteristics of the typical American form of 0. bifor- 
ata and becomes so numerous that a stratum from 2 to 10 feet 
in thickness and spreading over miles and miles of extent is 
made up almost entirely of the full grown shells of this fossil, 
and so certain is this layer to be found where the proper 
hight is reached that it is regarded by geologists as a most 
reliable land-mark. When the " 0. biforata stratum" is found 
the geologist knows that he is near the top of the Cincinnati 
beds. 
O. biforata did not die out after thus becoming so numerous, 
but continued in diminished numbers through the Lebanon 
beds, the uppermost member of the Cincinnati grouj) and on 
up through both the Clinton and Niagara groups — the former 
with a thickness of 50 and the latter with a thickness of 350 
feet, or a combined thickness of 400 feet. This added to the 
300 feet which constitute the Lebanon beds, the 425 feet of 
Cincinnati beds, and the 50 feet of Pt. Pleasant beds (now rec- 
ognized as Trenton) gives us a total combined thickness of over 
1200 feet of limestone and shale, and during the vast ages in 
