13 



Basal plates form an hexagonal disc which is nearly covered 

 with the end of the column. First primary radials large, about 

 as high as wide, three hexagonal, two heptagonal. Second 

 primary radials quadrangular, Trom three to four times as wide 

 as long. Third primary radials one-half larger than the second, 

 between two and three times as wide as long, pentagonal, ax- 

 illary, and on the distal side of each one adjoining the azygous 

 area bears a secondary radial that supports a free arm, and on 

 the proximal side two secondary radials, the last ones being 

 axilliary, which arrangement gives to each of these rays three 

 arms. In each of two lateral rays the third primary radial 

 supports two secondary radials, the last ones being axillary, 

 which gives to each of these rays four arms. In the ray oppo- 

 site the azygous area the free arms arise from each single 

 secondary radial, which gives to this ray two arms. There 

 are, therefoi-e. sixteen arms in this species. The arms are 

 composed of rounded cuneiform plates for about one-fifth part 

 of their length, above which tliey are composed of a double 

 series of interlocking plates and gradually flatten toward the 

 superior ends. 



There is a single regular interradial in each area and it con- 

 nects with the plates of the vault. The first azygous plate is 

 in line with the first primary radials and fully as large as any 

 of them. It is followed in the second range by three plates 

 and these unite with the plates of the vault. 



The vault is convex. Proboscis not exposed in our speci- 

 men. 



This is the twelfth species described from the Keokuk Group 

 that bears sixteen arms, but it is readily distinguished from 

 each of them by its general form and by the interradial plates 

 and structure of the arms. It is a marked species and though 

 our specimen is small, it is quite distinct and not the young of 

 any described species. 



Found in the Keokuk Group, at Boonville, Missouri, and now 

 in the collection of F. A. Sanijjson, of Sedalia, Mo 



