148 PALEONTOLOGY OF ILLINOIS. 



are generally hexagonal, and rest upon the truncated upper 

 eide of the basal piece, while the other two are heptagonal, 

 and rest upon the base so as to occupy more or less defined re- 

 treating angles or notches at the sutures dividing the basal 

 pieces. Second primy radials hexagonal, pentagonal or quad- 

 rangular; while the third, upon which the first bifurcation 

 takes place, are pentagonal, hexagonal, or heptagonal. The 

 secondary radial pieces, which are sometimes, though rarely, 

 wanting, usually vary from one to two in each division of each 

 ray, and support the brachial pieces, excepting where other 

 divisions take place upon intervening pieces. 



Anal plates varying in number in the different species ; the 

 first being usually nearly or quite as large as the first radials, 

 with which it also corresponds in resting directly down upon 

 the basal pieces. Interradials equally variable in number, ac- 

 cording to the species, and to some extent in different individ- 

 uals of the same species; the first is nearly always larger than 

 the others, and rests between the superior lateral sloping sides 

 of the first radial pieces. 



Arms varying in number from twelve to thirty or more, and 

 with rare exceptions all simple after becoming free; — always 

 composed, excepting at their bases, each of a double series of 

 small interlocking pieces, which support tentacles along their 

 inner sides. 



Vault composed of variously formed pieces, sometimes in 

 part swelling out into more or less prominent nodes, or even 

 short spine-like projections, either terminating in a slender, 

 more or less elongated, sub-central proboscis, having the open- 

 ing at its summit, or with a simple sub-central opening, with- 

 out a proboscis. 



Surface of body plates ornamented with radiating costse, 

 nodes or granules, or sometimes nearly or quite smooth. 



As in other allied types, the visceral cavity is known to 

 have been (in some species at least) provided with a curious 



