622 GEOLOGICAL SURVEY OF IOWA. 



pressed tuberculiform plate within the axil of each pair of 

 arms, and four somewhat larger ones around the central 

 plate ; two of them corresponding to the anterior ray, and 

 two others to the antero-lateral rays, while at the summit 

 are no large plates corresponding to the postero-lateral rays. 

 Arms beyond their bases, and column, unknown. Surface 

 beautifully radiating granulose-striate. 



This species differs from A. tuberosus in its generally more depressed form, and in 

 the smooth plates of the dome beyond the superaxillary plates, which are not con- 

 spicuous. There is no protrusive ridge on the anal side, and the plates are of different 

 form. The first brachial plates are larger, and their lower sides participate in the 

 concavity of the base. 



Geological position and locality. In Keokuk limestone : Green county, 

 Illinois. 



GENUS CYATHOCRIMJS (MILLER). 



GENERIC FORMULA. 

 Basal plates, 5. 

 Subradials, 5. 



Radials, 1 large and 2 small X 5. 

 Anals, 2 or 3. 



THE base is composed of five plates equal in size and shape, 

 forming altogether a regular pentagon with concave sides, 

 depressed in the middle for the attachment of the column, 

 with a central round or pentapetalous opening. Subradial 

 plates regularly alternating with the basal plates ; four of 

 them hexagonal, the angles at the base sometimes scarcely 

 salient : the fifth plate is heptagonal, being truncated above 

 by an anal plate, and sometimes separated on its upper 



