Observations on the Unification of Geological Nomenclature. 279 
Among the Cephalopoda, Nautilus is a living genus, though it 
should be considered doubtful as to identification in such remote ages; 
Oyrtoceras occurs in every group to the Coal Measures; Trochoceras 
to the Devonian; ZLituctes to the Niagara; Hndoceras to the Hudson 
River; and Cyrtocerina to the Black River Group. 
Among the Lamellibranchiata, Yellinomya occurs in the Black 
River, Trenton, Hudson River, and. as high as the Devonian; and 
Eopteria if distinct trom Huchasma is peculiar to it. 
Among the Crustacea, Beyrichia occurs in every group to the Coal 
Measures; Lichas occurs in the Chazy, Black River, Trenton, Hudson 
River, Niagara, and Lower Helderberg; Jllenus, Cerauerus, and 
Encrinurus, in the Chazy, Black River, 'Trenton, Hudson River, Clinton 
and Niagara; Harpes and Asaphus in the Chazy, Trenton, Utica Slate 
and Hudson River; QOlenus in the Hudson River; Yriarthrus in the 
Utica Slate; Remopleurides in the Chazy and Trenton; Ampyzx in the 
Chazy; and Hndymionia, Harpides, Holometopus, Megalaspis, Ni- 
deus, Ogygia, Shumardia, and Telephus, are peculiar to it. 
Of the fifty-seven genera that are thus supposed to have commenced 
their career in this group, twenty did not pass beyond it, but ten of 
these are Graptolitide, a family that commenced an existence in the 
Potsdam, and reached the climax of its evolution in this group, and 
declined gradually, thereafter, though the largest known form existed 
in the Hudson River Group. Eight of the remaining ten belong to 
the Trilobites, an order that flourished in the Potsdam, and was in the 
height of its evolution in this age, but continued to survive until near 
the close of Palzeozoic time. Seventeen of the genera passed into 
higher groups, but not beyond the Lower Silurian; ten became extinct 
in the Upper Silurian; five in the Devonian; four in the Carboniferous; 
and one is supposed to have survived all vicissitudes to the present time. 
It is connected specifically with higher groups by Maclurea atlan- 
¢ica, which passes up into the Chazy; and by Leptena sericea, which 
occurs inthe Chazy, Black River, Trenton, Utica Slate, Hudson River, 
and Clinton. 
The limits of the group are still a subject of discussion, but the 
existenve of it between the Calciferous and Chazy, seems to be firmly 
established. 
The Chazy Group.—In 1842, Prof. Emmons defined this group 
under the name of the “Chazy limestone.” The name is derived from 
the town of Chazy, in New York, where he found it well exposed be- 
tween the Calciferous and Birdseye limestone, and having a thickness 
