A, i Observations on the Unification of Geological Nomenclature, 287 
Among the Brachiopoda, Trematospira to the Hamilton. 
Among the Gasteropoda, Phragmostoma to the Hamilton. 
Among the Cephalopoda, Ascoceras passes up to the Clinton. 
Among the Lamellibranchiata, Sedywickia and Orthonota pass to the 
Carboniferous; Anodontopsis to the Devonian; Lschyrinia to the Clin- 
ton; and Anomalodonta, Cuneamya, Cycloconcha, Angellum and 
Orthodesma are peculiar to it. 
Among the supposed Annelida, Spirorbis extends to the Coal 
Measures; and Nereidavus and Walcottia are peculiar to it. 
Among the Crustacea, Cythere passes up through the Coal 
Measures, and Proetus to the Carboniferous. 
Of the 43 genera (without including tracks) thus enumerated, as 
commencing in this group, 19 do not pass beyond it; six become ex- 
tinct in the Upper Silurian, five in the Devonian, and the remaining 
eleven pass up into the Carboniferous. 
With this group, we close the Lower Silurian Formation, because we 
have here the greatest break, stratigraphically and paleeontologically, 
that occurs from the base of the Potsdam to the top of the Lower Helder- 
berg, and because it approaches nearer the line of division established 
by Murchison than any other thus far discovered. Wherever the 
Hudson River Group has been examined upon the continent, the 
superimposed rocks are unconformable with it, and no passage beds 
exist. In the western States the Niagara Group succeeds it. In the 
eastern States it is succeeded by the Medina and Clinton Groups before 
the Niagara is reached. On the Island of Anticosti, however, where 
the Hudson River Group has a thickness of 950 feet, it is succeeded 
by rocks resting conformably upon it, and with no apparent physical 
gap between them, although there is a sudden paleontological break. 
Of the 121 species known to Prof. Billings from this group, 80 suddeuly 
disappear below the break, 41 only passing upward where they are as 
suddenly joined by 45 species unknown in the Lower Silurian strata. 
This hiatus or dividing line between the Lower and Upper Silurian will 
be more distinctly realized when we reflect, that from the Potsdam, out 
of 62 genera, 46 passed to higher groups, and 20 of these passed beyond 
this line, and yet, when all the genera from the Lower Silurian are 
considered, making 280, only 120 passed this line, notwithstanding the 
latter number came into existence in the Trenton, Utica Slate and Hud- 
son River Groups, which are so intimately connected, and immediately 
precede this dividing line. 
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