566 THE CRINOIDEA CAMEEATA 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. 



Remarks. — 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 Plati/ceras 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 Plati/ceras 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 is worthy of note tliat tliis 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 ^Iso on most of the specimens of Scytalocrinus rohistus 

 (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 &ny 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 prej^ed either 

 on the Ophiurid, or on the Gasteropod, as suggested by the earlier writers. 



* See Synopsis of Amer. Carbon. Calyptraidse. By Charles R. Keyes. Proceed. Acad. Nat. Sci. Pliila, 

 1890, p. 154. 



