Observations on the Unification of Geological Nomenclature, 275 
The Gasteropoda are represented by Bellerophon, Platyceras, Pleu- 
rotomaria and Straparollus, which occur in every group as high as the 
Coal Measures. Pleurotomaria canadensis is known as a species to 
pass up into the Calciferous. By Holopea, which occurs in every 
group to the Warsaw in the Subcarboniferous; by Ophileta which oc- 
curs in the Calciferous, Quebec, Trenton and Galena; by Straparollina, 
which occurs in the Quebec and Black River Groups; and by Paleac- 
mea which alone is peculiar to it. 
The Cephalopoda are represented only by Orthoceras as above men- 
tioned. 
The Annelida are represented by Serpulites, which is accredited to 
the Chazy, Trenton and other groups as high as the Coal Measures. 
and by Salterella, which occurs in the Trenton. 
The Crustacea are represented by Agnostus, Conocephalites, Micro- 
discus, and Paradowxides, above mentioned; by Leperditia, which oc- 
curs in the Calciferous, Quebec, Chazy, Trenton, Utica Slate, Hudson 
River, Clinton, Niagara, and on to the Carboniferous; by Bathyurus, 
which occurs in the Calciferous, Quebec, Chazy, Black River and Tren- 
ton; by Amphion and Arionellus, which occur in the Calciferous, 
Quebec and Chazy; by Bathyurellus, Bathynotus, Crepicephalus, 
Pthychaspis, Olenellus, and Menocephalus, which occur in the Quebec; 
by Jilenurus, which occurs in the Calciferous; and by Aglaspis, Ag- 
raulos, Anopolenus, Chariocephalus, Conocoryphe, Pemphigaspis, So- 
lenopleura, and Triarthrella, which are peculiar to it. 
It thus appears that less than one third of the genera found within 
the whole range of rocks, which intervene between the metamorphosed 
Huronian series and the Calciferous, that have been included in the 
Potsdam group, are unknown in later times ; more than one third pass 
up into higher groups, but not beyond the Lower Silurian; of the re- 
mainder, a few became extinct in the Upper Silurian, a few in the 
Devonian, and the rest amounting to about twenty per cent. occur in 
the Carboniferous, and some in later formations. 
Or, in other words, more than two thirds of the genera known from 
the Potsdam occur in higher groups, and it is linked specifically with ° 
the succeeding Calciferous as well as graduating into it as above stated. 
There is nothing in the thickness, structure or fossil contents as it 
spreads over the States from the Appalachian mountains, that demands 
a subdivision beyond the provisional one of Upper and Lower Potsdam, 
and within the mountain districts, where it is found in such enormous 
thickness, it has not been separated into groups by satisfactory defini- 
