NIAGARA GROUP. 



177 



CRINOIDEA OF THE MIDDLE SILURIAN PERIOD. 



The Crinoidea of this period are confined to a very moderate thickness of strata, being limited 

 to the Clinton and Niagara groups, and not a single species being known either in the Medina 

 sandstone or Onondaga-salt group. Notwithstanding this limited range, the number of species 

 already known as having existed at this period is twice as great as from all the lower silurlan 

 strata. This proportion, moreover, is greatly increased, when we survey the entire extent of 

 the rocks of this period, if we may judge from the great number of species occurring in some 

 western localities. It should not be forgotten, also, that the whole number described in the 

 succeeding pages, ^vith two or three exceptions, have been found at a single locality of very 

 limited extent. Other exposures of the same rock, equally extensive, have afforded not more 

 them two or three species ; and in many localities, scarcely any thing more than fragments of 

 columns have been seen. These facts show very conclusively the habits of this class of animals, 

 which always congregate in great numbers within very limited areas ; while other portions of 

 the same strata, though presenting no essential variation of character, are almost entirely 

 destitute of them. In this instance, notwithstanding the whole line of exposure of the shale of 

 the Niagara group, west of the Niagara river, and eastward to Wolcott in Wayne county, 

 presents no essential difference in its geological features or mineral composition, we still find 

 the animals of this class, mostly restricted within a small area, in the vicinity of Lockport. At 

 this point, and the vicinity, the shale is covered by a heavy bedded encrinal limestone, composed 

 almost entirely of the broken columns of these animals, with an admixture of small corals, 

 and rarely a few other fossils. In the natural exposures, as well as the artificial excavations, 

 this rock is far better developed in this neighborhood than at any other place along the entire 

 escarpment. 



This period is not only more prolific in these animals than the lower silurian period, but even 

 much more than any portion of the succeeding strata, whether silurian or devonian, within the 

 State of New-York. At the present time we do not know half as many species, in the entire 

 series from the Onondaga-salt group to the Chemung group inclusive, as are known in the 

 Niagara group alone. This condition and degree of development in this family of fossils, and 

 probably also to a greater or less extent in the corals, corresponds with formations of the same 

 age in Europe. 



Mr. MuRCHisoN, in his introductory remarks to the Crinoidea of the Silurian System, says, 

 " In the mean time I may state that the Crinoidea are more abundant in the Wenlock forma- 

 tion than any other member of the Silurian System, by far the greater number of the forms 

 described having been formed in the limestone of that age near Dudley." 



The variety of form, appendage and ornament of these fossils, is proportionally much greater 

 than in the species of the lower strata. The normal or pentapetalous base is still the prevailing 

 [Palaeontology — Vol. ii.] 23 



