22 



hundred and seventy-seven species, twenty-four have been re- 

 ferred to Eretmocrinus, which genus we have elsewhere shown 

 is based upon an association of peculiar specific characters, 

 one or more of which belongs to ordinary Batocrinus, and each 

 of which occurs in species undoubtedly referable to Balocrinus. 

 Beside, Eretmocrinns is only found associated with Balocrintif, 

 from the Burlington to the Warsaw Group. 



Three species only have been described from the Kinderhook 

 Group or rocks below the Burlington and each of these pos- 

 sessed twenty arms, and only two from the Kaskaskia Group, 

 one of which possessed eighteen and the other twenty arms. 



In the Burlington Group they are found with eight, ten. 

 twelve, thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, 

 nineteen, twenty, twenty one, twenty-two, twenty-three, twenty- 

 four, twenty-six and twenty-seven arms. In all the range of 

 arm formulas from eight to twenty- seven only nine, eleven 

 and twenty-five are unknown. The most numerous species 

 possessed eighteen, twenty and twenty- two arms. 



In the Keokuk Group they are found with twelve, fourteen, 

 fifteen, sixteen, seventeen, eighteen, nineteevi, twenty, twenty- 

 one, twenty-two, twenty-four, twenty-five, twenty- six, twenty- 

 eight, twenty nine, thirty, thirty-two, thirty-three, thirty-six 

 and foi-ty arms. The most numerous species posses.sed sixteen, 

 seventeen, eight(>en, twenty and thirty-two arras. In all the 

 range of arm foi-iiiulas from twelve to forty only thirteen, 

 twenty-three, twenty .seven, thirty-one, thirty four, thii'ly five 

 and thirty-.seven to thirty-nine are unknown. 



In the Warsaw and St. Louis Gi-oups they are fouiul witli 

 seventeen, eighteen, twenty and forty ai-ms. The most numer- 

 ous species possessed eighteen and twenty arms. 



It will be noticed that while no species has been described 

 from the Burlington Group with twenty five arms, some liave 

 been described with that numbei- in the Keokuk. It must lie 

 b irne in mind too that some species have been imperfectly de 

 scribed without giving the arm formula, and it may be that the 

 few missing links in the arm formulas, from eight to forty, 

 can ()!■ will be supplied. Any one studying these forms must 

 become convinced that the arm formula, in all cas(>s, in tliis 

 genus, is of specific im])ortance. After having examined many 



