586 GEOLOGICAL SURVEY OF IOWA. 



a brachial plate, and on its inner side a pentagonal third 

 supraradial, which supports two brachial plates; thus giving 

 four arms from each supraradial, or eight from each ray : 

 these all being equal, make forty arms altogether. The arms 

 of each ray are separated at the base by a long narrow plate, 

 and those of the division of the ray by a similar plate, or by 

 the extended edges of the brachial plates, sometimes ap- 

 pearing as a double plate. First interradial plates much 

 smaller than the first radial, supporting two smaller hexa- 

 gonal or subhexagonal plates ; and above these a third range 

 of two still smaller plates, which support the outer division 

 of the adjacent rays. First anal plate hexagonal, and of the 

 same size as the right postero-lateral first radial, supporting 

 two second anals, one heptagonal and the other hexagonal : 

 third range of two similar smaller plates, with a small qua- 

 drangular plate inserted centrally between the four. A similar 

 central one in the fourth range supports the second supra- 

 radial plates of the outer divisions of two adjacent rays. 



SURFACE of plates ornamented by rounded somewhat tu- 

 berculous ridges which radiate in one or two to each side of 

 the plates, sometimes bifurcating, and frequently interrupted 

 and becoming nodulose. Surfaces between the second radials 

 and interradials, and between the supraradials at the base 

 of the arms, deeply excavated. 



SUMMIT above the axil of each set of arms composed of 

 numerous minute plates ; the space above with larger and 

 smaller plates, which are raised in tubercles and sometimes 

 short spires or angular ridges, and, near the margins of the 

 plates, smaller pustules. Plates of proboscis proper, coarsely 

 granuliferous. 



In this species the character of the surface is very distinct from that of any of the 

 preceding species; and though bearing a general resemblance to A.proboscidialis, f. 13, 

 it is remarkably distinct not only in this, but in the number of arms. In the number 

 and arrangement of plates of body and base of arms, this species corresponds with 

 A. multibrachiatus ; but the form of body, the proboscis and surface markings, are 

 yery distinctive. 



