280 PALEONTOLOGY OF ILLINOIS. 



Eadials pentagonal, nearly twice as wide as long, depressed on 

 their lower margins to correspond with the depressions in the sub- 

 radial plates. They are constricted across the middle, and project- 

 ing on their upper margins, with a distinct suture between them 

 and the brachial plates. 



First brachial quadrangular, the second pentangular, sharply an- 

 gular above, giving support on its upper sloping angles to the first 

 division of the rays. 



The arms on the three anterior rays, after their first division on 

 the second radials, di\ide at least four times, and one of them, the 

 left antero-lateral ray five times, the division in every case taking 

 place on the outer branch, as in Zeacrinus, while the inner branches 

 continue single to their extremities. The divisions generally occur 

 on the sixth, eighth, twelfth or sixteenth plate, making ten arms to 

 two of these rays, which is probably the normal number, and eleven 

 to the other. On the posterior rays, these divisions take place on 

 the fourth, sixth and eighth plates. This gives an arm formula of 

 fifty arms, as the normal number for this species. 



First anal plate hexagonal, about as large as the smallest sub- 

 radials, second and third rather smaller than the first, and all 

 arranged as usual in this genus. They are all depressed at the 

 angles, though not quite so much as the other plates of the body. 

 Column unknown. 



Position and locality : Upper part of the geodiferous shales of the 

 Keokuk group, one mile below Keokuk. 



Nos. 100 and 185 of Mr. L. A. Cox's collection. 



POTERIOCKINUS ASPERATUS, Worthen. 



P1.XXV1II, Fig. 2. 



Poteriocrinus asperatus, WORTHEN, February. 1882. 



Bulletin No. 1, of the Illinois State Museum of Natural History, p. 12. 



Body depressed, forming a low saucer-like cup below the summit 

 of the radial plates. Base depressed, and basal plates concealed by 

 the upper columnar joints. Subradials small, their lower angles 

 concealed in the basal concavity. 



Eadials about twice as wide as long, pentagonal, without any well 

 defined suture between them and the first brachial plates. 



Brachials two, about the same size as the radials, the first quad- 

 rangular, and the second pentangular, giving support above to ihe 

 first divisions of the rays. 



