XVI INTRODUCTION. 
That part of my work already completed in detail, shows a more perfect similarity with 
the Lower Silurian strata of Europe than we had heretofore supposed ; and though there 
is not a large number of identical species, the analogy of others is quite interesting and 
satisfactory. 
In our extreme lower formations, we have found little that can be positively identified 
with European deposits ; still, the Potsdam sandstone, our oldest fossiliferous rock, appears 
to hold, in the scale of formations, a similar place with the Ungulite grit of St. Peters- 
burgh,* which is there charged with fragments of Obolus, giving it the almost micaceous 
aspect which the Lingula does to its analogue in New-York. The paucity of organic forms 
at this period, and in the two succeeding formations, would lead us not to expect a very 
complete analogy between groups of this age in countries widely separated from each other. 
We have, nevertheless, in these extremely ancient deposits, a few forms which are 
peculiar, and sufficiently characteristic to identify the strata in which they occur. These 
forms do not appear to have been recognized elsewhere ; or if known, no great importance 
has been attached to them. It is probable, however, that in Europe, as in many parts of 
the United States, the Calciferous sandstone and Chazy limestone are not well developed, 
and consequently their typical fossils do not exist. We find ourselves, therefore, forced to 
commence our comparison with European formations, from the Trenton Hmestone; in 
which, thus far, the forms of Ophileta,t Scalites, Raphistoma and Maclurea, which are 
typical of the preceding strata, have not been observed. 
When we compare our species of the lower groups with those of Europe, as made known 
to us, first, by the publication of the Silurian System of Mr. Murcutson, and subsequently 
in the work already cited, by the same Author, E. pr Verneuin and Count von Kryser- 
Line, as well as from other sources, we find a very instructive and interesting analogy. 
The Geological Report of the Ordance Survey of Ireland, by Capt. Portitocx,i has like- 
wise shown us other forms identical with our own; and we are able to prove, from this 
author, that the zoological relations of the lower strata in Ireland are more analogous to 
those of New-York and the United States, than are those of the same age in England and 
* See Geology of Russia in Europe and the Ural Mountains, by Ropericx I. Murcuison, Epouarp DE VER- 
NEvIL and Count A. von Kryserxine, Vol. i, pp. 27 & 27*. 
t At page 11 of this Report, I have remarked that the Ophileta complanata, from its usually rare occurrence and 
obscure characters, cannot be regarded as of much importance as a characteristic fossil. Mr. Vanuxrem has since in- 
formed me, that in a late geological excursion in the vicinity of Lake Champlain, he has found this fossil in great 
numbers and excellent preservation, holding the same position as elsewhere in the State. It is probably, therefore, 
entitled to more consideration than I had before attached to it. 
t Geological Report on Londonderry, and Parts of Tyrone and Fermanagh, by J. E. Porriock, F.R.S., F.G.S., 
&ce. Dublin & London, 1843. 
