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 Batocrinns, and each 

 of which occurs in species undoubtedly referable to Balocrinus. 

 Beside, Eretmocrinus is only found associated with Balocrimif, 

 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, nineteen, twenty, twenty- 

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

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

 and forty arms. The most numerous species possessed sixteen, 

 seventeen, eighteen, twenty and thirty-two arms. In all the 

 range of arm formulas from twelve to forty only thirteen, 

 twenty-three, twenty-seven, thirty-one, thirty-four, thirty-five 

 and thirty -seven to thirty -nine are unknown. 



In the Warsaw and St. Louis Groups they are found with 

 saventeen, eighteen, twenty and forty arms. 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 have 

 been described with that number in the Keokuk. It must be 

 bDrne 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 or will be supplied. Any one studying these forms must 

 become convinced that the arm formula, in all cases, in this 

 genus, is of specific importance. After having examined many 



