as stated, in Ohio Pal., vol. 2, p. l(>i. an arm formula of 4-[ ^^i- 

 2 13 + 4 IC) arms, or -4+3+2+4+4^17 arms. Wo are inrlincd to 

 think that two species are described here under one name; bid speci- 

 mens from that locality are very poor, and those which we have, 

 possess only sixteen arms, and we cannot throw any additional li.yht 

 upon the snbject. The reference of the species, however, to A(/m-i- 

 coi'riiuis is very doubtful. 



A(j(iv!roi-viiiii^ )}0<1nlosiii< from the Keokuk (irouj) has 17 arms. 

 The arm formula is 4+4+^+4+3. And .+ iiidcdiliniisi has IS 

 arms. The arm formula is 4-|4+4-i3+3. This species is one of 

 the lartfest. has the most numerous arms and belonu's to the last of 

 the race. 



^Ye hp.vi' thus taken, briefly in review, all the sijecies of .{(/(irico- 

 ci-inn^ that have been described. They belong to the Chouteau. 

 Burlington and Keokuk Groups. Names have been given to forty 

 two species. All of these have been illustrated except five, three of 

 which are believed to be good and valid sj);'<'ies anil Ihe other two 

 may be classed as doubtful, because they were named thirty six years 

 ago and have never bei-n illustrated, and some authors have said they 

 are synonyms. All we can say is that we have not identitied them .in 

 our collections. It is very easy for some to say. that one species is a 

 synonym for another, but the student will find that good authors 

 rarely make a synonym, and. if one occurs, it is under such cir- 

 cumstances that ordinary ])rudeiice will not yiiard ai^aiust it. We 

 have seen ten-armed species desc-ribed by Hall, from the Burlington 

 Group, twcdved-armed species described by Hall from the BnrLing- 

 . ton Group, twelve-armed species described by Meek &- Worthen fr«m 

 the Burlington (irouj). and twelve-armed sju'cies described by Hflll 

 from the Keokuk (iron]), all of which have been finely illustrated, and 

 occur, in Iowa and Illinois, classed as synonyms for Aii<iricnfrliitis. 

 (uiicricdiiiis. which has never been described, hut which isafourteen- 

 armed sjiecies, as shown by the illustrations, and has never been 

 known to occur, except in the Keokuk Group of Temiessec. Such 

 erroneous classifications are without any discrimination; and usually 

 without examination, but there is no way of preventing the iJi'eten- 

 sions. 



We would again call attention to the distribution of the species in 

 the different Groups of rocks. The species from the Chouteau lime- 

 stone are small and bear either nine or ten arms. Those from the 

 Burlington Group are larger than those from the Chouteau, but are 



