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 ses¬ 
sile in the young state, and free as they approach maturity, while others 
are provided with a column. 
The Genus Platycrinus occurs in this iieriod '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 ( Le- 
pocrinus, Conrad ; Apiocystites, Forbes) are of generic types identical with 
those of the Niagara period. 
The following lists of genera of Crinoidese and Cystideoe, 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. 
CRINOIDEiE AND CYST!DINE 
CRIXOIDE.E AND CYSTIDEiE 
of the 
of the 
Clinton aiul Niagara groups. 
Lower Helderberg group and Oriskany sandstone. 
Closterocrinus. ..... 
1 sp. 
Glyptocrinus [?]....* 
1 
Homocrinus . 
2 
Homocrinus. 
1 sp. 
Glywtaster . 
1 
Mariacrinus. 
8 
Tiiysanocrinus . 
4 
Platycrinus . 
4 
Dendrocrinus . 
1 
ICHTHYOCRINUS. 
1 + 1? 
Aspidocrinus. 
2 
Lyriociiinus . 
1 
Edriocrinus . 
2 
Lecanocrinus .. 
4 
Brachiocrinus. 
1 
Saccocrinus . 
1 
CORON OCRINUS . 
1 
Macrostylocrinus . . 
1 
Eucalyptocrinus .... 
3 
Stepiianocrinus ...... 
2 
Caryocrinus . 
1 
Meeocrinus . 
1 
Heterocystites ...... 
1 
Anomalocystites .... 
1 
Callocystites . 
1 
Spii^erocystites . 
1 
Apiocystites . 
1 
Apiocystites 
( = Lepadoceinus), 
1 
Hemicystites . 
1 
Pal^easter . 
1 
Protaster ? . 
1 
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. 
