11 



There liavc lieen descrihed. from the Keokuk Group, eight twelve- 

 armed species, all of which liave been properly illustrated. They 

 are as follows: A. cratasKs, A. clefinns, A. eris, A. indianensis, A. 

 sjdeiideus, A. ^primjeri, A. tiihcrosu^i and .4. fiif/nrinm. These spe- 

 cies are widely different from each other and clearly distinct. A. 

 eris is from Richfield, Ohio, and, at the time of its original defini- 

 tion, was referred to the Waverly Group: but the rocks are decidedly 

 above the Waverly Group, and are now known to belong to the 

 Keokuk. 



A. iiihe)'()iii(fi, Hall, is properly the type of the genus. Prof. Hall 

 published Troost's Mss. definition of the genus (Geo. Sur. Iowa, 

 560). and said it was founded upon A. fubcrosus, proposed by Troost, 

 which he said had been later described by Roemer, under the name 

 of AiiipbonicrinuH aiiierlcanus. Prof. Hall then proceeded to define 

 ,1. iiilio-osiis (Geo. Sur. Iowa, 617), and very fully described a 

 twelve-armed species, which is very common about Keokuk, Iowa, 

 and of which we have examined more than one hundred good speci- 

 mens. The species described by Roemer, under the name of Am- 

 phonicrinus americnmift has very little resemblance to it, and is, 

 as we understand his illustration, a fourteeii-armed species, and We 

 have specimens of it collected at the typical locality in Tennessee, 

 which bear fourteen arms. The Mss. name, A. fubcrosus. 

 Troost, has no validit}'. because he did not define the species 

 and because no one else has published what he said about 

 it. Prof. Hall used the specific name fuberosKs supposing 

 that he was applying it to the same form to which Troost had ap- 

 plied it and to which Dr. Roemer had given the name aincricanus; 

 but he was applying it to quite a marked and different species 

 which he carefully described, and which must bear his name, be- 

 cause a catalogue name does not preoccupy a word or give it any 

 force as a specific name. The name is. therefore, ^4. tuberosus, Hall, 

 and not A. tiibrrosits, Troost. Prof. Hall followed the definition of 

 the genus, with the description of the species ^4. hiillafiis. and pub- 

 lished the definition of A. Iiiberi)f<iis farther on in the book, but 

 that does not affect the question of the tj'pe of the genus, for he 

 very clearly set forth A. fubcrosus as the typo. 



The mistake of calling A. fubcrosus a synonym for A. (iiiiericanus 

 has, probably, been carried into the literature of the subject, because 

 so few copies of Bronn's Lethea Geognostica, in which Roemer jDub- 

 lished his A. americaiius, ever reached America. We have never 



