110 
PALAEONTOLOGY OF NEW-YORK. 
tains two species ; while the one above cited occurs everywhere in this limestone, as far west 
as Iowa on the west of the Mississippi. Probably nowhere in the world are there so many 
beautiful and really splendid specimens of this coral to be found, as in the western United 
States. It is not a little interesting that this coral, though occurring in such immense numbers 
at this period, has never been found in any stratum of more recent date. 
The Genus Stromatopora, in a species allied to or identical with S. concentrica , occurs at 
this period, and is found in masses of one to two feet in diameter. Numerous branching corals, 
allied to Ch.zetetes or Favosites, occur in the lower part of the limestone, but they really 
belong to other genera. In the Genus Limaria, or allied genera, we have several species; 
while in the true Bryozoa, we have the genera Retepora, Polypora, Fenestella, &c., de¬ 
veloped in great numbers of individuals, though of comparatively few species. 
Finding it impossible to refer to established genera many species occurring in this group, I 
have constituted new genera to receive them. The practice of referring species of ancient corals 
to existing genera, or to those of modern geological periods, is often or always attended with 
uncertainty, unless the comparison of species be made directly. When.we find, for example 
that a large part of the corals of the Upper Silurian are of distinct genera from those of the 
Lower Silurian strata; and that again many new generic types are introduced at the Devonian 
period, while nearly all the older types have disappeared, we can form some idea of the im¬ 
propriety of referring Silurian corals to genera typical of the Oolite or Chalk periods, or of 
modern seas. Still it is to some extent almost impracticable to depart entirely and at once from 
this custom, particularly as we have as yet no systematic description of fossil corals, or any 
authoritative limitation of the application of generic names. The comparison of specimens with 
figures is often not satisfactory ; and since we have no accessible authentic collections, it is 
not always easy to decide upon questions of generic and specific identity and difference. 
In the following species, the figures have all been made from actual specimens; and no 
figure elsewhere represented, although sometimes regarded as identical, has been copied, or 
made the basis of any of those here given. The geological position of these specimens is 
well established ; and therefore the great point of their order in time will always be reliable, 
whatever changes may be made in other respects. 
With the exception of some of the smaller corals, all these species have been collected with 
my own hands ; and the isolation of this group in the western part of New-York, where it is 
separated from the upper limestones by one thousand feet of thickness and twelve miles in 
width of the Onondaga-salt group, which is there almost entirely non-fossiliferous, sufficiently 
proves that no intermixture of corals from succeeding formations have found their way into the 
collection. 
