102 The American Geologist Fcb.isoo. 
which it was being deposited this particular species continued 
to exist. 
As a contrast to the life history just given is that of the fossil 
Stropho'menapla7io-convexa,a\QO found in the Cincinnati group 
at Cincinnati,' at an elevation of 300 feet above low water. 
While this species lived long enough to spread itself over tlie 
entire bottom of the sea in which the Cincinnati group was 
deposited, it is only found through a few feet in thickness of 
the 800 feet of limestone and shale that are included under 
this general name. OvthisretTorsa is another fossil brachiopod 
that makes its appearance in the same limestone at the hight 
of 475 feet above low water, but is only found for a foot in 
thickness — when, so far as known, it disappears entirely. Yet, 
judging from the character of the ro(^k or the other fossils 
associated with this, there was no physical change of any 
kind that marked its appearance or disappearance. 
The trilobites furnish us with a stranger story of life and 
death — death not only of the species but of the whole family, 
so that to-day they have not a living representative in the 
world. They made their appearance in the Primordial period, 
increased in the Lower Silurian, were most numerous in the 
Upper Silurian and "after there had been, under a succession 
of genera, more than 1700 species, came nearly to an end 
with the Devonian — the old genera being all extinct and only 
three new ones appearing in the Carboniferous to close off this 
prominent PaLneozoic type." (Dana's manual of geology, page 
289). Although these lingering forms carry the life history of 
the trilobites to the close of Palaeozoic time, yet the fact 
remains that the end of the Carboniferous age saw their final 
extinction. 
The question naturally presents itself. What caused the 
extinction of species in past geological ages? Let me say 
here that no one great cause need be sought, but rather a vast 
number of slight and almost imperceptible ones. A case once 
came under my own observation that well illustrates to what 
slight changes we must look for information on this subject 
Ten years ago I made a trip to the Tuscarawas river four 
miles south of this city, (Akron, 0.) for the purpose of obtain- 
ing some specimens of a river mussel, TJnio clavus^ not found 
'"The Cincinnati Group," Prof. Orton Geological Survey of Ohio, 
vol. I, page 393. 
