ioo Cincinnati Society of Natural History. 



diversity in the descriptions of the various species will be 

 observed. This is due to the various plans followed by the 

 different writers. There is a lamentable lack of uniformity 

 in describing many fossils that often renders any comparison 

 difficult. No division of families has been attempted in the 

 present enumeration. The following " Key," although im- 

 perfect, may be useful in locating the genus of a given speci- 

 men. 



Key to Genera. 



Under basals, 5, generally well developed ; basals, 5 ; arms, 

 10-24 or more; column sub-pentagonal. 



1. — Reteocrinus. 



Under basals small or absent ; basals, 5 ; arms, 10-30 ; 

 column round. 



2. — Canistocrinus. 



Under basals small or absent ; basals, 4 ; arms, 10; column 

 square. 



3. — Xenocrinus. 



Under basals, 5; basals, 5; arms, 20; column round. 



4. — Ptychocrinus. 



Under basals absent; basals, 5 ; arms, 20; column round 

 or pentagonal. 



5. — Glyptocrinus. 



Basals, 4; arms, 20; column square. 



6. — Mariacrinus. 



Basals, 5, unequal; radials, 5, irregular; arms, 10; column 

 round or pentagonal. 



7. — Heterocrinus. 



Basals, 5, unequal; radials, 5, irregular; arms, 10; column 

 round, tripartite. 



8. — Ectenocrinus. 



Basals, 5, irregular; radials, 5, irregular; arms, 10; column 

 pentagonal, pentapartite. 



9. — Ohiocrinus. 



Basals, 5, regular; radials, 5, nearly regular; brachials 

 many; arms, 10 (?) ; column pentagonal. 



10. — Iocrinus. 



2 



