494 THE CEINOIDEA CAMERATA OF KORTH AMERICA. 



cavity is formed by the basals, radials, and the lower portions of the first 

 interbrachials, which are stretched out horizontally, and form the flat bottom ; 

 the first costals constitute the sides, while the second costals and the upper 

 portions of the interbrachials deflect outward and slightly upward, and form 

 the rim upon which the calyx rests ; the latter plates are a little convex, 

 those within the convexity are flat. Suture lines well defined. 



Basals concealed by the column. Eadials nearly as long as wide, the 

 lower margins deflected to form the columnar depression. First costals 

 quadrangular, nearly twice as wide as long, bending abruptly upwards on. 

 their lower margins ; the second larger and throughout wider than the first, 

 often larger even than the radials. First distichals quite variable, in some 

 specimens almost twice as large as in others ; they are succeeded by a single 

 cuneate plate, or by two pieces, the edge of the one overlapping the other. 

 Arms ten ; contiguous to the third plate above the axillary ; stout, but 

 shorter than in any of the preceding species. First interbrachials sub- 

 ovate, very large, sometimes twice as large as the radials; they rise to 

 the middle of the first distichals, where they are followed by two long, 

 very narrow pieces, which reach up to the level of the arm openings. 

 The first anal, which is narrower and higher than the radials, supports 

 three large plates, the middle one narrower than the two outer, and there 

 are three others in the next range. Posterior oral somewhat excentric, 

 leaning to the posterior side, very large and strongly nodose ; it is sur- 

 rounded by nine or ten plates consisting of the smaller orals, which are less 

 tumid and of but half its size, two somewhat smaller radial plates, and three 

 to four small, almost flat, supplementary anal pieces. The radial plates near 

 the outer margin of the disk are somewhat larger than the smaller orals. 

 Interambulacral plates not numerous, there being rarely more than five to 

 each side. Anal area wide and flat ; the anus placed midwaj' between the 

 arm regions and the summit of the posterior oral ; the opening directed 

 laterally. Column very small compared with the size of the species; joints 

 of almost uniform width ; axial canal pentangular. 



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

 and at the same horizon in Illinois and Missouri. 



Type in the Illinois State collection at Springfield. 



Remarks. — This species resembles A. Wortheni Hall, in general outlines, 

 and has a similar flattened anal area, but the latter is a much larger species, 



