23 



thousand specimens belonging to this genus, we are free to 

 say, that in no case have we found two specimens having a 

 different number of arms, that agreed in other characters, and 

 hence the number of arms, as shown, in the calyx, before the 

 arms became free, must rank in the first degree in determin- 

 ing the specific characters and in entitling the form to a specific 

 name. 



As no crinoidal species has ever been known to pass from 

 the Kinderhook to the Burlington, or from the Burlington to 

 the Keokuk, or from the Keokuk to the Warsaw or St. Louis, 

 or from the St. Louis to the Kaskaskia, we have forty- three 

 species above mentioned that may be distinguished by the 

 number of arms alone. 



We do not claim to have been original m laying stress upon 

 the arm formula, in determiniing species, in this genus, for 

 Prof. James Hall, in the contributions to the Palaeontology of 

 Iowa, published in 1859, very properly defined the arm formula 

 as of specific importance, in all the Actinocrinidoe, he described 

 in that work. 



The radial series furnished the characters for the determin- 

 ation of species no less important than the arm formula. In 

 some species the first secondary radials are axillary, and in 

 other species the second secondary radials are axillary. No 

 two specimens possessing these characters can belong to the 

 same species. It will be seen at once that the specimen hav- 

 ing two secondary radials will have ten more radial plates 

 within the calyx than the specimen having only one secondary 

 radial, and a further examination will disclose the fact that 

 the interradial areas are correspondingly different and the 

 plates different in number or in size. The first tertiary radials 

 may bear arms or they may be axillary, and the second tertiary 

 radials may bear arms or they may be axillary, but no 

 two specimens can belong to the same species, if they differ 

 in these respects. That is, if one specimen has a single teritary 

 radial and another has two teritary radials they cannot belong 

 to the same species. And, again, while the number of arms 

 in two species may be the same, yet the radial series will be 

 entirely different; for example, one specimen may have three 

 arms in each of four rays and four arms in the other making 



