178 Cincinnati Society of Natural History. 
am unable to refer it to any genus yet described, and hesitate to found 
a new one upon a singleexample. The specimen is accurately repre- 
sented in the figures which show the arrangement of the plates very 
perfectly on the dorsal side, while, owing to a silicious coating, the 
basal portion of the opposite side can not be deciphered. The figures 
are drawn of the exact size. The column is round, and tapers rapidly. 
The peculiar character of this anomalous fossil is the presence of a 
single arm, originating between two large plates which form the apex 
of the body on that side. Seven plates of the arm are shown. Near 
it, upon the left side, as shown in the figure, is a small tubercle, evi- 
dently formed by valvular plates now silicified so as to obscure their 
arrangement. I figure this specimen in order to bring it to the notice 
of those interested in the remarkable series of Echinoderms which the 
Trenton rocks of Canada, and their equivalents in Kentucky, have 
afforded. I especially invite correspondence from all workers upon 
these fossils, and should be glad to borrow, and will freely loan any — 
specimens offering instructive points. 
AGaArRicocrinus, Troost. Cat. Proc. Am. Ass., 1850. 
AGARICOCRINUS CRASSUS, NOV. sp. 
Formula as usual in this genus. The type specimen is represented 
in three views on Plate V. Fig. 1, basal view ; fig. la, azygous side ; 
fig. 16, summit view, the latter somewhat distorted by the irregular 
manner in which the specimen is drawn. This species is characterized 
by the very heavy character of its whole structure, the great thickness 
of the plates both of the radial and interradial series, and by the 
massive character of the apical dome plates. The aperture of the 
azygous side is surrounded, as may be seen in fig. la, by a ring of 
regularly arranged pentagonal plates. Beyond these, the arrange- 
ment of the plates of this area follow no regular order, nor do they 
agree in number in different specimens. The smaller plates of the 
apical series follow the same plan, having no regular order of arrange- 
ment, nor being of the same number in the different specimens of the 
species. <A striking character is the arrangement of the pelvic plates 
in such a way as to render the lower surface of the species slightly 
convex instead of deeply concave as is usual in this genus. From 
rocks of the Keokuk group, sub-carboniferous, Tenn. The rocks from 
which these fossils were obtained, present a vertical section of trom 
200 to 600 feet. Fossils of the Burlington Group, or those closes 
allied, occur in the lower part at its greatest thickness. 
