THYSANOCRINID_E. 205 
Idiocrinus ventricosus W. and Sp. (nov. spec.). 
Plate XVIII. Figs. 9a, b. 
A very small species. Calyx as wide as high; height of dorsal cup about 
equal to that of the tegmen; the former bowl-shaped, the cup obtusely 
pyramidal. Plates smooth; the radials and costals somewhat longitudinally 
convex, causing a small depression of the interbrachial spaces. Suture lines 
slightly grooved. 
Infrabasals extremely small, and completely covered by the column; 
placed at the bottom of a small circular cavity, formed by the lower ends of 
the basals. Basals of moderate size, their lower ends incurving and forming 
the sides of the concavity, their upper angles slightly bending upwards. 
Radials once and a half as large as the two costals together, and twice as 
wide as long; three of them heptagonal, the two posterior ones hexagonal. 
First costals quadrangular, much narrower than the radials, and three times 
as wide as long; the second of nearly the same width as the first, but longer. 
Distichals narrower and shorter than the second costals. Interbrachials large, 
subelliptical; that of the anal side a little wider, and slightly truncating the 
posterior basal. Oral pyramid convex, a very little tumid, extremely large 
for the size of the species, perfectly closed at the summit, and the inter-oral 
sutures obsolete. The outer surface of the pyramid is covered with well 
defined radiating ridges, which proceed from the middle of the plate to the 
outer margins, increasing in height and width as they pass outward. Ten of 
these ridges are prominent, and project outward around the circumference, 
thus giving to the plate the aspect of a ten-rayed star. The inner floor is 
excavated centrally, and there are five deep grooves passing out in a radial 
direction. The interambulacral plates long, slender and cuneate, attached 
with their sharp upper ends to the inner margins of the orals. They project 
outward so as to form at their sides open spaces for the reception of the 
ambulacra. Other parts unknown. 
Horizon and Locality.— Upper part of Niagara group; near St. Paul, Ind. 
Type in the collection of Wachsmuth and Springer. 
Remarks. — The oral pyramid of this species, which is found occasionally 
detached from the body, was regarded by Miller, Benedict, and ourselves, as 
probably representing the ventral structure of a Pisocrinus. This appeared 
quite plausible, as it was expected from analogy that in Pisocrinus, as in the 
