24 G ■ PALAEONTOLOGY OF ILLINOIS. 



unequal pieces, which do not project out beyond the circum- 

 ference of the upper extremity of the column. Subradial 

 , plates irregular in form and size, one of them apparently 

 quadrangular, two pentagonal, one hexagonal, and one hepta- 

 gonal, or octagonal. First radial pieces nearly twice as wide 

 as long, two of them heptagonal, two hexagonal, and one 

 apparently pentagonal ; all more or less arcuate laterally ; 

 second, third and fourth radials (also the fifth in the anterior 

 ray) not differing greatly in size, wider than long, and hexa- 

 gonal in form. Fifth radials (sixth in the anterior ray) of the 

 nearly same size as tBe others, heptagonal in form, and sup- 

 porting on their sloping sides the secondary radials. 



First interradials nearly half as large as the first radial 

 plates, heptagonal and hexagonal in form, and each supporting 

 two or three smaller pieces in the second range; above these, 

 three or four pieces are seen in the next range, which is as far 

 up as they can be counted in the specimen before us, though it is 

 probable the number increased at the same rate for several 

 ranges above. Anal pieces unknown, excepting the base of 

 the first one, which is very small, and as usual in this group, 

 rests in a sinus in the upper side of the large odd subradial 

 piece; secondary radials above the first divisions of the rays on 

 the fifth and sixth primary radial pieces, continued nearly on 

 a line with the primary radials below, without diverging so as 

 to leave any room for interaxillary pieces. In one of the poste- 

 rior rays seen, a second bifurcation takes place on the fourth 

 piece, beyond which the pieces in this ray are not preserved in 

 our specimen. 



Column round, comparatively thick at its connection with 

 the base, from which it tapers towards the lower extremity; 

 composed of extremely thin segments, fitting together by inter- 

 locking crenulations formed by the radiating striae; its central 

 cavity subcircular, or obscurely pentagonal, and about one- 

 third as broad as the transverse diameter of the column. 



