Observations on the Unification of Geological Nomenclature. 283 
Among the Crustacea, Jsochilina occurs in the Trenton, Hudson 
River, and Medina, and Cytherina and Cytheropsis in the Trenton. 
Of the 25 genera thus enumerated as commencing an existenc2 in 
this group, only three are peculiar to it; eight become extinct in the 
Trenton and Hudson River; five in the Upper Silurian; four in the 
Devonian; and five in the Carboniferous and Permian. This group is 
especially distinguished for the wonderful evolution of the Cephalopoda 
which it presents. One fifth of all the genera belonging to this class 
in the Paleozoic rocks came into existence in this group. One genus 
commenced and terminated its existence, and the family Orthoceratide 
that commenced its career in the Potsdam passed its period of greatest 
development here, though some species survived until the age of the 
Coal Measures. 
The Trenton Group.—This group was named from Trenton, Oneida 
county, New York. The limestone, at Trenton Falls, where it is over 
100 feet in thickness, was called the “ Trenton limestone,’ long prior 
to the use of the words in a geological sense. In 1838, Lardner Van- 
uxem referred to the Trenton limestone, but it was not until 1842 that 
he and Prof. Emmons so defined the group as to fully establish it. 
At Trenton Falls, it consists, in the lower part, of a dark-colored, 
fine-grained limestone in thin layers, separated by black shale or slate, 
which forms the great mass through which the creek has worn its 
channel, and in which are all the falls; the upper part is a gray, coarse- 
erained limestone in thick layers. It is finely developed in Lewis, 
Oneida, Herkimer and Montgomery counties, where its thickness 
is about 300 feet, and still more extensive in Clinton and Jefferson 
counties, where it has a thickness of 400 feet. It also occurs in other 
counties of the State. In Vermont there are three narrow outcrops, 
consisting of black schistose layers, associated with slaty seams of 
limestone, and occasional argillaceous matter, having a thickness of 
about 400 feet. It has an extensive geographical distribution in Cana- 
da. The Montreal, Bay St. Paul, and Ottawa sections have, each, a 
thickness of 600 feet. The sections in western Canada, on the Trent 
river, and at Collingwood, have a thickness of more than 750 feet. It is 
much thinner farther west in Wisconsin and Minnesota. In Missouri 
it is more than 300 feet thick. In eastern Tennessee it consists, at 
the base, of a highly ferruginous sandy limestone, having a thickness 
of 700 feet, followed with flaggy limestone and calcareous shale, 800 
feet thick, and this again by variegated marble 300 feet. In middle 
Tennessee, and central Kentucky, the thickness is about 500 feet. The 
