25 



AGARICOCR1NUS ILLINOISENSIS, n, sp. 



Plate I, Fig. 21, basal view: Fig. 22, summit view; Fig. 23, azy- 



gous side view. 



Species medium or rather below medium size. Calyx very- 

 low, moderately concave in the region of the basals and first 

 primary radials, then gradually rounds over on the radial 

 series, which are produced nearly on a level, while the plates 

 curve up in the interradial areas. The concavity at the place 

 of attachment of the column is about on a level with the dor 

 sal side of the last secondary radials. The outline of the calyx 

 is pentagonal with slightly concave sides for the depressed in- 

 terradial areas. Plates thick, smooth or granular. Ambulac- 

 ral openings elongated, directed upward and not visible in a 

 basal view. Column medium size. 



Basals form an hexagonal disc that is nearly or quite covered 

 by the column. The first primary radials are elongated, trans- 

 versely concave to correspond with the basal concavity, three 

 hexagonal and two heptagonal. Second radials much smaller 

 than the first, quadrangular, and from one-half wider to twice 

 as wide as long. Third primary radials shorter and wider than 

 the second, pentagonal, axillary, and support on each upper 

 sloping side three secondary radials, the last one of which 

 commences the double series of arm plates, but the inner half 

 connects with a plate belonging to the vault that separates the 

 ambulacral canals, and, therefore, does not belong to the free 

 arms. There are only ten arms in this species, and ten arm 

 openings to the vault. The arms evidently consist of the 

 usual double series of plates united by a zigzag suture. 



There is one regular interradial in each area. It is elongated 

 and has nine sides. It rests between the short superior lateral 

 sides of the first primary radials, separates the primary and 

 secondary radials and connects its superior, narrow elongation, 

 by a short truncated end, with a plate belonging to the vault, 

 midway between the arms, at the top of the calyx. The first 

 azygous plate is in line with the first primary radials and of 

 the same general form, though longer and narrower, and of 

 course heptagonal. It is followed by three plates, in the sec- 

 ond range, the lateral ones are short and heptagonal, the mid- 

 dle one is elongated, hexagonal and unites its superior trun- 

 cated end, with a plate belonging to the vault, midway between 

 the arms, at the top of the calyx. The vault is low, most 

 convex centrally, and the pentagonal outline, at the margin, 

 —4 



