566 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
in width they grow shorter and cylindrical, while the others relatively 
increase in size. | 
Horizon and Locality. — Keokuk group; Indian creek, Montgomery Co., 
and Canton, Washington Co., Ind. 
Types in the collection of Wachsmuth and Springer. 
ftemarks. — Of this large and beautiful species we obtained at Indian 
creek and Canton over forty specimens, most of them in excellent preserva- 
tion, with the arms attached; and it is very remarkable that nearly one half 
of them have either a Plutyceras attached to the tegmen, or a specimen of 
Onychaster between the arms and coiled around the anal tube. This, so far 
as we know, is the first instance in which a Platyceras has been found in 
contact with a Crinoid with a long anal tube; in all cases heretofore noticed 
the Crinoid had an anal opening directly through the tegmen, and the 
Gasteropod was fastened invariably with the anterior portion of the shell 
over the opening.* This led to the supposition, for which there seemed to 
be good reasons, that the Mollusk obtained its nourishment, in part at least, 
from the excrements of the Crinoid. This, however, was impossible in the 
case before us, where the anal tube, with the anus at the distal end, extends 
out far beyond the tips of the arms, and, so far as observed, bends abruptly 
to one side, so that neither the opening nor the refuse matter could have 
been in contact with the Mollusk. | 
In more frequent association with this Actinocrinus is the Onychaster, and 
it 1s worthy of note that this species of Ophiuran is rarely found by itself, 
Nor has it been observed at Indian creek on any of the other Crinoids, while 
at Canton it appears also on most of the specimens of Scytalocrinus robustus 
(Hall), a species with a large ventral tube, and the anus located far down at 
the anterior side; but with this exception we have not seen it on any other 
species. The fact that this Ophiurid is only found associated with certain 
species, and there always under similar conditions, and the frequency of this 
occurrence, would seem to indicate that the position between the arms of 
these Crinoids was its favorite resting place, in which it either found protec- 
tion, or some special facility for obtaining nourishment. Nobody, however, 
who is acquainted with the anatomy of the Crinoids, and their mode of liv- 
ing, will entertain for a moment the notion that the Crinoid preyed either 
on the Ophiurid, or on the Gasteropod, as suggested by the earlier writers. 
* See Synopsis of Amer. Carbon. Calyptraide. By Charles R. Keyes. Proceed. Acad. Nat. Sci. Phila, 
1890, p. 154. 
