Observations on the Unification of Geological Nomenclature. 285 
16 terminated their career within it. Streptorhynchus filitextum, Asa- 
phus gigas, Calymene callicephala, Rhynchonella capax, and numer- 
ous other species, pass up into the Hudson River; while Zygospira 
modesta, and Strophomena tenuistriata pass on into the Upper Silurian. 
The Utica Slate Group.—This Group was named from its exposure 
at Utica, New York, and was defined, in 1842, by both Lardner Van- 
uxem and Prof. Emmons. It extends from New Jersey, across the 
State of New York, into Vermont, and passing under Lake Champlain 
enters Canada, where it spreads over a large area to the north and east, 
and extends westerly from Lake Ontario to Lake Huron. It is through- 
out a dark colored slate, loaded with carbonaceous matter. It is inter- 
stratified with thin bands of limestone, and passes into the Trenton 
Group below by gradual interstratification. In some places the lam- 
ine are coated with a thin film of anthracite, and in other places sili- 
cious slate occurs of red, green, brown and black colors. In New 
York it reaches a thickness of 250 feet; in Vermont, 100 feet; in Can- 
ada, 510 feet; and in Pennsylvania, 400 feet. It thins out westerly, 
and while it separates the Trenton and Hudson River Groups at Cin- 
cinnati, on the Ohio river, it has lost its character of a black slate, and 
is composed of blue calcareous shales and marls, with interstratified 
thin limestones. and apparently forms mere passage beds from the 
Trenton to the Hudson River Groups. The distribution of the genera 
which commence an existence in this group is as follows: 
In the vegetable kingdom, Cyathophycus and Discophycus are pe- 
culiar to it. 
Among the Brachiopoda, Zeptobolus and Schizocrania pass up into 
the Hudson River. And among the Crustacea, Plumulites passes up 
into the Hudson River. 
Of the five genera thus supposed to commence their existence in this 
group, two do not pass beyond it, and the other three become extinct 
in the Hudson River. 
While many of the Trenton species pass through it, yet there are a 
number which are characteristic of it, among which is 7'riarthrus becki, 
a species that follows the group in its extended geographical distribu- 
tion, and changes of character, but is not known in either older or 
younger strata. 
The Hudson River Group.—This group was named from the ex- 
posure on Hudson river, in New York, and first defined, in 1842, by 
Lardner Vanuxem. In New York it consists of shales, shaly sand- 
stones, sandstones, slates and thick-bedded grits. The slates and 
