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PALEONTOLOGY OF NEW-YORK. 
CRUSTACEA OF THE CLINTON AND NIAGARA GROUPS. 
The Trilobites of the Clinton group form no very conspicuous feature among its organic 
remains, and in many parts of the group they scarcely appear at all. In the Niagara group, 
however, these fossils are among the most important and characteristic forms oAhe lower strata, 
and are every where seen in this position in the State of New-York. On examining westward, 
however, the shaly portion of the group is scarcely conspicuous; and the limestone, having 
greatly increased in thickness, predominates over all the other deposits of the^Middle Silurian 
period. In this limestone, trilobites are of rare occurrence, and not characteristic of the forma¬ 
tion ; while there is a vast increase in the number of corals, compared with the same rock in 
New-York. It is not until we trace this group many hundreds of miles to the west and south¬ 
west, that we find it assuming in some degree the characters by which it is distinguished in 
New-York, and it again becomes marked by the presence of trilobites and crinoids. 
Although we observe that several species of trilobites are typical of the shale of the Niagara 
group from its commencement in central New-York, yet we rarely find entire specimens east 
of the Genesee river. From this line onward to Lockport in Niagara county, j^ere appears to 
be a constantly increasing number of individuals, and a greater degree of perfection in the 
specimens obtained. From this circumstance, and from the fact that in the western part of the 
State crinoids flourished in great numbers at the same time, we may infer that many of the 
fragments (consisting mostly of bucklers and caudal shields of trilobites), found in the thinning 
eastern extension of the shale, have been drifted from localities farther west. 
The ten species already known in the Niagara group belong to nine distinct genera, giving 
two species only of the Genus Proetus. If to these we add four distinct species of the 
Clinton group, not known in the Niagara period, we have fourteen species, with the addition 
of one more genus, Acidaspis, fragments of which have been seen in the Niagara shale. In the 
succeeding strata of the Silurian system we have scarcely another genus to add to those of this 
period, and the number of species in all the Upper Silurian strata will scarcely equal those of 
the Lower Silurian period. 
In addition to the Trilobites, we have a single species of Beyrichia, and one of Cytherina, 
ascertained; and probably a second species of the former genus, which is not satisfactorily 
determined. 
