11 



these two plates would represent the first plate, as usually found 

 in this genus, and the two plates would be larger than a first 

 primary radial and it would reduce the number of plates, in the 

 azygous area, to thirteen. We are of the opinion that this fea- 

 ture is abnormal and that another specimen may show a single 

 plate instead of two. In the nest range there are three plates, 

 in the uext four, in the nest three, and in the next two, which 

 separate the tertiary radials and unite with the plates of the 

 vault. 



The vault is depressed, convex, covered with polygonal plates, 

 and bears a small subcentral proboscis. The ambulacral openings 

 are directed upward and visible in a summit view. No ovarian 

 pores discovered: 



This species is a cousin of A. speclahilis, above described, if 

 not nearer related. It will be noticed that this is a shorter form, 

 and radial ridges more prominent than in A. spectabilis, but such 

 differences are not of specific importance when the arm formulas 

 are the same. The differences between the two that may be of 

 specific value are these: In this species each regular interradial 

 area has seven plates, two of which connect with the plates of the 

 vault; in A. specta1)ilis none of the plates in the regular interadial 

 areas connect with the vault, and, in each of two of them, there 

 are only five plates. In this species, supposing the division of 

 the first plate, in the azygous area, to be normal, there are thir- 

 teen plates in the azygous area, the last two being comparatively 

 large and connecting with the plates of the vault; in A. specia- 

 hilis there are sixteen plates, in the azygous area, only one of 

 which connects with the plates of the vault. We have frequently 

 found as much difference, in the number of interi-adials, in differ- 

 ent specimens, in the same species, as we find in these two spe- 

 cies; but not under the same circumstances. It is the difference 

 in the number of plates, coupled with the fact that the interradial 

 areas are connected with the vault in one case, and not in the 

 other, that we are inclined to think is of specific value. There 

 does not seem to be anything abnormal about our specimens ex- 

 cept the division of the first azygous plate. 



Found in the Burlington Group, at Burlington, Iowa, and now 

 in the collection of A. Albers. 



