PALAEONTOLOGY OF IOWA. 593 



supporting on each side two or three brachial plates. This 

 structure is modified on the two divisions of the supraradial 

 plates of each antero-lateral ray, and more extremely mo- 

 dified in the postero-lateral rays, where there is an additional 

 plate introduced on each side above the divisions of the 

 supraradial plates, giving the requisite number of arms to 

 that ray. This gives four arms from the anterior ray, five 

 from each antero-lateral ray, and six from each postero- 

 lateral ray ; 



4 



giving = 5 5 = 28 arms. 

 ~6~ ~6~ 



This arrangement is, however, [sometimes] modified as shown 

 in the diagram, where one of the postero-lateral rays has 

 seven arms, and the adjacent antero-lateral ray four arms. 



First interradial plates large, tuberculiform, ten-sided, 

 supported laterally and upon the upper lateral edges by the 

 first and second radials and the first supraradial plate, and 

 succeeded by two small second interradials. Third range 

 consisting of a single interradial, supporting subdivisions of 

 the supraradial plates of two adjacent rays. First anal plate 

 larger than the first radials. Second anal plates three, tu- 

 berculiform, the central one largest : above these is a small 

 intercalated plate, and an arch of three which support the 

 supraradial plates of the postero-lateral rays. Arms and 

 proboscis unknown beyond their base. Dome composed of 

 numerous small tuberculiform plates. Surface beautifully 

 granulose striate. 



The basal and first interradial plates, and the first and 

 second anal plates, are conspicuously tuberculiform ; the 

 plates of the other ranges being much smaller : the three 

 radial plates occupy scarcely more than one-third the length 

 from the base to the arms. 



This species differs in the number of arms, as well as in form, from any other 

 species described in this place. 



[lowA SURVEY.] 75 



