10 



third range, five in the fourth range, three in the fifth range, 

 and one in the sixth range that separates the tertiary radials and 

 extends to the vault plates. 



The vault is depressed, convex, covered with small 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 distinguished by its general form from all other 

 twenty-armed species heretofore described, and no comparison 

 with any of them is necessary. 



It was found in the Burlington Group, at Burlington, Iowa, 

 and is now in the collection of S. A. Miller. 



ACTINOCBINUS S0BRINU8, n. Sp. 



Plate III, Fig. 4, azygous side; Fig. 5, opposiie vieto. 



Species below medium size. Calyx obpyramidal one-half wider 

 than high, truncated only the size of the column, most rapidly 

 spreading toward the free arms. Ambulacral openings directed 

 upward and not visible in a basal view. Ila*lial ridges strongly 

 defined. Interradial areas flattened in the lower part and de- 

 pi'essed between the arms, where the plates unite with those of 

 the vault. Plates convex. 



Basals form a low cup three times as wide as high. First pri- 

 mary radials wider than long, three hexagonal and two heptag- 

 onal. Second primary radials about half as large as the first, 

 hexagonal, wider than long. Third primary radials about as large 

 as the second, three hexagonal, two heptagonal, axillary and sup- 

 port on each upper sloping side a single secondary radial, which 

 is axillary and supports, on each upper sloping side, a single ter- 

 tiary radial, which gives to each ray four arms. There are, there- 

 fore, twenty arms in this species. 



There are seven plates in each regular interradial area, one in 

 the first range, two in the second range, two in the third range, 

 and two in the fourth range, which separate the tertiary radials 

 and unite with the plates of the vault. There are fourteen plates 

 in the azygous area. The first is in line with the lower part of 

 the first primary radials, and rather more than half as large as 

 one of them, tt is followed by a single plate in the* second 

 range of about the same size. Suppose this to be abnormal, then 



