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PALEONTOLOGY OF NEW-YOKK. 



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 of the 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, there 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 equs.1 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. 



