30 



four arms. Tliere are, therefore, eighteen arms and eighteen am- 

 bulacra! openings to the vault in this species. The arm formula 

 is 4+4+2+4+4. The irregularity of the radial series is very re- 

 markable, and yet the calyx is as symmetrical as it is in many 

 other species, and the same features occur in several specimens. 



The interradial areas are different, but they are all cut off from 

 the vault. In the area, on each side of the azygous area, there 

 are three plates, one followed by two, in the second range. In 

 each of the other areas there are four plates, one in the fiist 

 range, two in the second range, and an elongated plate in the 

 third range. In the azygous area there are eight plates. The 

 first plate is in line with the first primary radials and nearly as 

 large. It is followed by three plates, in the second range, three 

 ia the third range and one in the fourth range, which is cut off, 

 at a distance from the vault, by the union of the tertiary radials. 



Vault highly convex or obconoidal, with slightly concave de- 

 pressions toward the interradial areas. It is covered with smooth, 

 polygonal plates, some of which are convex, and bears a subcen- 

 iral proboscis. The ambulacral openings are visible in a summit 

 view. No ovarian pores discovered. 



This species is essentially different, in its structure and general 

 form, from all other eighteen-armed species, and agrees with B. 

 levis, only, in the fact that they both have smooth plates and bear 

 eighteen arms. B. levis has no proboscis, this species has one, 

 the form of calyx and vault is different and the structure of the 

 radial series above the third primary radials is different, in the 

 two species, as well as the regular and azygous areas. The two 

 species cannot be mistaken for each other. It is hardly necessary 

 to compare it with any other described species. It belongs to a 

 long list of species having smooth or finely granular plates, but 

 that is a feature of minor importance. 



Found in the Burlington Group, at Burlington, Iowa. The 

 specimen illustrated is in the collection of Mr. A. Albers; others 

 are in the collection of S. A. Miller. 



