THE 
AMERICAN GEOLOGIST 
Vol. V. MAY, 1890. No. 5. 
OBSERVATIONS ON THE KEOKUK SPECIES OF AGARICO- 
CRINUS. 
By C. H. Gordon, Keokuk, Iowa. 
[Read before the Iowa Academy of Science, Sept. 5, 1889.] 
The Keokuk beds have thus far furnished six species refer- 
red to this genus, of which three — A. Americanus Roemer, ^. 
lohitfieldi Hall, A. ivortheni Hall — may be found in collections 
made at and about Keokuk. We have had access to a laro-e 
number of these forms and from a somewhat careful study- 
have become impressed with the need of a revision of this 
genus. As between A. americanus Roemer, and A. vjortheni 
Hall, the specific distinctness seems well established. The es- 
sential differences, as shown by Hall', occur in the second and 
third radials and in the interradial areas. In A. ivortheni Hall, 
the first interradials are much shorter than in A. americanus 
Roemer, thus bringing the second interradicds much lower and 
giving them a proportionately greater prominence in the 
structure of the calyx. In consequence of thi.s they are 
brought in contact with the second radials whose upper lateral 
angles are truncated to receive them, thus giving these plates, 
a hexagonal form, while in the typical form of A. avicricanus 
Rcemer the long ///•.s^ interradial and the quadrangular second 
radials are very characteristic. Moreover in the A. ivortheni 
iGeol. Surv. Iowa., Vol. I, Pt. 2, p. 620. ' ' 
