514 THE CEINOIDEA CAMERATA OF NORTH AMERICA. 



pressed, the bottom concave to the middle of the first costals, which together 

 with the first interbrachials and second anal plate are geniculated, or ab- 

 ruptly beat upward, and swollen so as to form a circle of hemispherical 

 nodes around the margin of the concavity ; the upper portions being almost 

 vertical. Ventral disk depressed pyramidal, with a short conical spine at the 

 summit. Surface of plates throughout the calyx finely granulose, and suffi- 

 ciently convex to bring out the suture lines. 



Basals hidden from view, occupying the bottom of the column concav- 

 ity. Eadials about as wide as long, all hexangular in outline. First 

 costals slightly larger than the radials and quadrangular; the second not 

 longer than the first, but often considerably wider, and provided with an 

 obtuse mesial ridge, which connects with the prominences upon the first. 

 The distichals from the second up are free. The arm facets have a slightly 

 upward tendency, and those of the same ray are placed at right angles ; 

 arms a little longer and more slender than in A. brevis. Interradial 

 spaces wide and somewhat depressed in the upper portions, a little con- 

 tracted at the arm regions. First interbrachial large, curving abruptly 

 upward ; followed by two elongate pieces, and a small quadrangular one, 

 the latter occupying the arm regions. The interambulacral plates consist of 

 five or six pieces. The two lower anals vary considerably in size ; the first 

 being in some specimens not longer than the radials, in others almost twice 

 as long. In the latter case the first takes part in the circle of nodes which 

 form the lower margin o£ the calyx, in the other it occupies together with 

 the radials the lower concavity, and the second anal is nodose and forms 

 a part of the circle ; the interbrachials at the sides of the second anal are 

 elongate and widest in the middle. The next row consists of three smaller 

 plates, which are succeeded by numerous irregular still smaller ones, forming 

 an almost flat area. Anus directed laterally and placed low down, a little 

 above the arm openings. Posterior oral large, high conical or subspinous ; 

 the four others are scarcely elevated. The radial dome plates somewhat 

 projecting and moderately convex. 



Horizon and Locality. — Upper Burlington limestone, Burlington, Iowa. 



2'ype in the Museum of Comparative Zoology. 



Remarks. — The circle of tumid plates at the lower margin of the calyx 

 distinguishes this species from all others. 



