202 PALEONTOLOGY OF ILLINOIS. 



form, resembling therein Troostocrinus and Met&blasiw. 

 O. conicus differs from O. fusiformis in the form of its ventral 

 side, which is hemispherical, in the latter conical. Besides it 

 has shorter ambulacra, and a less number of pinnules. 



O. conicus has the form of an elongate inverted cone with 

 nearly straight sides; ventral side depressed, hemispherical. 

 Surface of the plates smooth, or in well preserved specimens 

 marked by indistinct concentric lines around the plates. Basal 

 portions occupying more than two-fifths of the general height 

 of the calyx, or fully one-half the length up to the radial lips. 

 Neither the interbasal nor basi-radial sutures are marked 

 by any sort of identation, and both are therefore rarely observed; 

 The interradial sutures, however, are slightly depressed, especi- 

 ally along their upper portions, where they form an obtuse re- 

 treating angle, thus giving to the section across the lips an 

 obscurely stellate outline. 



Radials or forked plates almost twice as wide as high; their 

 lateral faces nearly parallel; the lower sides in the three plates 

 facing the interbasal sutures angular, in the two others rounded. 

 The limbs of the radials extend to about one-half the length of 

 the ambulacra; they bend inward, and leave a comparatively 

 wide radial sinus. 



The interradials or deltoids are partly visible in a side view; 

 they are double-diamond shaped, constricted along the middle, 

 so as to appear almost like two separate plates. The plate at 

 the azygous side is slightly bulging, pierced by a large elliptic 

 anal aperture, which in three of our specimens is completely 

 closed by a disk, composed of a large number of very minute 

 irregular pieces, which rest somewhat deeper than the surround- 

 ing plates, leaving no trace of an opening. The oral opening is 

 sharply pentangular and moderately large much larger than 

 in O. stelliformis. It is covered in perfect specimens by com- 

 paratively large, very regularly arranged summit plates, con- 

 sisting of a central piece, and four large and two smaller proxi- 

 mals, from which irregular rows of covering pieces extend to 

 the ambulacra, arching over the food groves. 



Ambulacra short, more or less petaloid, only partly visible in 

 a side view. They extend laterally to the slit-like openings or 

 clefts, and present an even surface with the surrounding calyx 



