204 PALEONTOLOGY OF ILLINOIS. 



less extent, from the deltoids down to about half the height of 

 the basals, giving to the species an angular outline, by which 

 it is readily distinguished from 0. conicus. 



The surface ornamented with delicate concentric lines, finer 

 than those of the preceding species. The specimens are light in 

 color. 



Basals nearly as high as the radials to their lips; forming 

 jointly a subconical cup with slightly convex sides; angular 

 above for the reception of the radials ; two of the upper faces 

 moderately concave, the other three angular. 



Forked plates or radials elongate ; the prongs or limbs nearly 

 as long as the handle or bod}^; width across the lips almost 

 twice as great as at the two ends; the lateral faces as well as 

 their outer face gently and uniformly curving upwards and 

 downwards; the median line toward the sinus somewhat angu- 

 lar; the lips projecting, flanging outward so as to give to the 

 calyx up to the lips a reversed bell-shaped form. The sinus is 

 longer than in the preceding species, increasing in width up- 

 ward; its edge distinctly beveled. Interradial and basi-radial 

 sutures well defined. 



Deltoids long, about as high as the basals, the upper sides 

 deeply constricted near their upper end for the admission of a 

 large spiracle; the lower sides less than half the length of the 

 upper. 



Ambulacra long, somewhat wider than usual in this genus; 

 their sides almost parallel except at the distal end. They occupy 

 fully one-half the length of the calyx, and rest deeply within 

 the radial sinuses, which have steep sides, Lancet pieces visible 

 throughout their full length; the exposed part at the proximal 

 end equal to one-third the width of the plate. Food grooves 

 deep, especially near the mouth, however, when closed by covering 

 pieces these form conspicuous ridges, which extend inward as 

 far as the summit plates. Side pieces with a well marked 

 tubercle in the direction of the lancet piece, and superficial 

 grooves toward the pinnules, which rest within shallow pits. 

 The pinnules are composed of single joints, rather flat, scale- 

 like, upper and lower faces undulating. There are ten groups 



