LOWER HELDERBERG ROCKS. 



101 



part of the body. It may yet be found that these two forms are so nearly 

 identical in structure as to constitute one genus ; some species being sesr 

 sile in the young state, and free as they approach maturity, while others 

 are provided with a column. 



The Genus Platycrinus occurs in this period for the first time in 

 American Silurian strata ; and though this genus is very prolific in the 

 Carboniferous period, I am not aware at present of its occurrence at any 

 point between the Lower Helderberg limestones and the Carboniferous 

 limestone. 



The species of the Genus Mariacrinus compose by far the most con- 

 spicuous forms among the Crinoideans, both of the limestones and the 

 succeeding sandstone. Two species of Homocrinus and a cystidean (Ze- 

 pocrinus, Conrad ; Apiocystites, Foebes) are of generic types identical with 

 those of the Niagara period. 



The following lists of genera of Crinoideae and CystideaB, known in the Clinton and 

 Niagara groups, and in the Lower Helderberg group and Oriskany sandstone, afford the 

 means of comparison of these fossils in the different periods. 



There are, besides these, several species in both periods, which have thus far been determined only 

 by fragments of columns, etc., and which are not included in the above lists. 



