152 PALAEONTOLOGY OF ILLINOIS. 



third radial plates, above which the secondary radial, and 

 brachial pieces, curve abruptly outwards to the base of the 

 arms, so as to form, with the ventricose summit, a much ex- 

 panded visceral cavity, entirely above the basal and primary 

 radial plates. 



Base unknown; first radials unknown, excepting from some 

 remaining portions of their upper ends, which show that they 

 are comparatively large. Second radials very small, a little 

 wider than long, (those seen) irregularly pentagonal in form, 

 one of the sides being much shorter than the others. Third 

 radials as long as the first, and nearly one-third wider; the 

 only two visible in our specimen hexigonal in form, and each 

 supporting on its superior sloping sides two secondary radials 

 of about its own size. Each of these is surmounted by a some- 

 what larger second secondary radial, which in its turn supports 

 two first brachial pieces, each of which is succeeded by a sec- 

 ond, from which the free arms are given off. The two series 

 of secondary radials, and the four series of brachial pieces in 

 each ray, coming in contact laterally, leave no room for inter- 

 axillary and interbrachial pieces. Interradial plates two or 

 three, the first being about the same size as the second radials, 

 and hexagonal or heptagonal in form. Above this there is one 

 or two small pieces of variable size and form, over which the 

 secondary radials, and the lateral series of brachial pieces of 

 the rays on each side, connect all the way up to the free arms, 

 in such a manner as to leave no spaces for interradials, above 

 those just described. (Anal plates unknown.) 



Dome hemispherical, composed of pentagonal, hexagonal 

 and heptagonal plates of nearly uniform size, each of which is 

 provided with a spine-like tubercle. Proboscis central, or 

 nearly so. Arm openings twenty. Surface smooth or obscure- 

 ly graulose; small rather pointed tubercles are also seen on 

 the second and third radial plates, first interradials, and first 

 secondary radials. 



