57 



each side of the azygous area the distal side supports two sec- 

 ondary radials and the proximal side an axillary secondary radial 

 which supports, on one upper sloping side, two tertiary radials 

 and, upon the other, one, which gives to each of these rays three 

 arms. One of the lateral rays on the right side of some speci- 

 mens and upon the left side of other specimens is constructed in 

 the same way, and bears three arms. In each of the other two 

 rays the third primary radial supports, on each upper sloping 

 side, two secondary radials, which gives to each of these rays two 

 arms. There are, therefore, thirteen arms in this species. The 

 arm formula is 3+3+2+2+3, or 3+2+2+3+3. 



In the elongated specimens there is only one regular interradial 

 in each area, and it is below the secondary radials, but in the 

 broader forms, in some of the areas, there is an additional plate 

 that separates the secondary radials, in one or more of the areas, 

 and unites with the plates of the vault. There are nine plates in 

 the azygous area, in the elongated specimens. The first one is in 

 line with the first primary radials and of about the same size. It 

 is followed by three plates in the second range, three in the third 

 range, and two in the fourth range that unite with the plates of 

 the vault. In the shorter forms there are only eight plates in 

 the azygous area. 



The vault is much more convex or conical in the elongated 

 specimens than in the broader forms and it bears a nearly central 

 proboscis. 



This species will not be mistaken for any other thirteen- armed 

 species. Its affinities are with B. affinis above described and B. 

 verneuilianus. The basals form an hexagonal cup in B. affinis; 

 horizontally truncated below, a round cup, in this species, beveled 

 to the column, and a round cup, with a band at the bottom in B. 

 verneuilianus. There are more azygous plates in this species than 

 there are in B. affinis, and as many as there are in B. verneuilianus. 

 But the difference, which we regard as most important, is found 

 in the number of ambulacral openings to the vault. We think 

 the difference in form and in the regular and azygous areas would 

 not be of specific importance, without a corresponding difference 

 in the arm formica. 



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

 mens illustrated are in the collection of A. Albers, but both forms 

 are also in the collection of S. A. Miller, and in that of Win. F. 

 E. Gurley. 



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