20G PALEONTOLOGY OF ILLINOIS. 



support, on one of their superior sloping sides, a large brachial 

 piece, and on the other a secondary radial of near the same 

 size, which in its turn supports two brachial pieces, making 

 three arms to each of these rays; while in all the others two 

 brachial pieces rest directly upon the third radial, thus making 

 only twelve arms in the whole series. 



The first anal plate is about as wide as the first radials, but 

 a little longer, being longer than wide, instead of the reverse. 

 It is regularly heptagonal, and supports on each superior late- 

 ral sloping side, a smaller heptagonal piece ; while an elonga- 

 ted, coffin-shaped piece rests upon its short, superior, truncated 

 side between the two latter, and extends up, flanked on either 

 side by the first brachials, to the summit. Each of the inter- 

 radial spaces is filled by a single, ovate, octagonal piece, about 

 as large as the second and third radial pieces taken together. 



The vault is composed of rather large, somewhat regularly 

 arranged plates, which are very nearly flat, the smaller ones 

 all being on the anal side ; the proboscis is small, and located 

 nearer the anal than the dorsal side. Height to base of pro- 

 boscis, 0.50 inch; breadth, 0-58 inch; breadth of base, 0.23 

 inch. 



Fig. 14. This symmetrical little species is so nearly lite A. rotiin- 



C2^) dm, of Shumard QGeol. Report Missouri, p. 191, pi. A, 



c2 P) e^C"} O fig. 2, 2 a, 2b), that it might readily be confounded with 



&^/~yp^(~y^C it, although it is clearly distinct. In the first place it is 



^^Ov3[V">0 less distinctly rotund than that species, while it differs in 



0j^)^C)/ : ~\O0' k^g entirely destitute of secondary radials, excepting a 



^O O c^&J single one on one of the sloping sides of the third radial 



Aetinocrinus dodecadacty- piece in each posterior ray ; while in A. rotundus there are 



tos-(Naturaisize). tw0 f these pieces, one upon the top of the other, resting 



upon the superior sloping sides of each third radial. This difference in the 



arrangement of the parts gives twenty arms in Shumard's species, white in 



ours there are only twelve. Again, our species has but a single interradial 



piece in each space, while in A. rotundus there are from two to three. The 



anal pieces are likewise more numerous in Shumard's species than in ours, 



while its plates are more flattened, and its sutures less distinct. 



