170 PALEONTOLOGY OF ILLINOIS. 



Primary radials large ; the first and second hexagonal ; the 

 third or axillary one pentagonal. The lower portion of the 

 first radial and the interposed first anal plate somewhat 

 abruptly bent inward to form the basal concavity, while the upper 

 portions are slightly bent upward in such a manner that the 

 calyx rests upon the median portions of the plates. Second 

 radials somewhat smaller than the first, but much larger than 

 the third. Secondary radials 2x2x5, about as large as the pre- 

 ceding axillary. They support upon their nearly truncate upper 

 sides two rows of tertiary radials, of which, however, only the 

 lower ones take the form of radials, the others resemble closely 

 free arm plates. 



Arms short, the tips bending inwards; bifurcating soon after 

 they become free, and two or three times above. At their 

 bases in the calyx the arms are very robust, but they diminish in 

 size rapidly from each bifurcation upwards. They are composed 

 of a double series of low transverse pieces, longitudinally 

 arranged, and alternating. The pinnules are closely packed to- 

 gether and composed of short joints. 



Interradials 12 to 15 or more up to the arm bases, the lower 

 one as large or larger than the second radials, hexagonal, sup- 

 porting on its sloping upper sides two plates of the size of the 

 third radials; hexagonal or heptagonal according to whether 

 there are two or three pieces in the third range. The plates of 

 the third range resemble in size the secondary radials against 

 which they rest. They are of large size compared with 

 the plates of the upper ranges, especially those of the equa- 

 torial zone, which comprise the fifth and sixth ranges. Axygous 

 interradius much larger than the four regular ones. First 

 anal plate a little higher than the first radials, and narrower. 

 It supports three large plates, and above several other ranges 

 of considerably smaller pieces. There are six or more interax- 

 illaries, the lower one as large as the interradials of the third 

 range, the upper ones like the succeeding interradials. 



Vault composed of slightly convex plates, almost of uniform 

 size. Summit plates a little larger ; continuous in mature speci- 

 mens; never separated by the intercalation of small plates. 

 They consist of a central piece and six proximals, of which four 



