492 THE CEINOIDEA CAMERATA OF NORTH AMERICA. 



produce depressions at the interradial spaces. Bottom of the dorsal cnp 

 deeply excavated to the middle of the second costals, the distichals thus 

 forming the base upon which the calyx rests. All plates within the con- 

 cavity are flat ; while the distichals are more or less convex, and sometimes 

 covered with indistinct transverse angularities. 



Basals small, more deeply depressed than the surrounding plates, and 

 hidden from view by the column ; the axial canal moderately small and pent- 

 angular. Eadials more than twice as wide toward the iipper end than at the 

 lower ; the lower margins inflected to form the basal concavity. First costals 

 one half Avider than long, the upper face wider than the lower ; they are 

 quadrangular in outline, although frequently one or both upper angles are 

 slightly truncated by the second interbrachials. Second costals as large as 

 the radials or larger, six to eight-sided. Distichals 2X2, twice as long 

 as the arm plates, interlocking with their fellows of the opposite division, 

 and with the arm plates to the third row. Arms two to the ray, long, 

 very heavy in the lower portions, but gradually tapering until quite thin 

 toward the extremities. Interbrachials three ; the first comparatively^ short, 

 often not reaching the top of the first costals ; the two succeeding ones un- 

 usually long and extremely narrow, rising from within the basal concavity to 

 a level with the arm bases ; these plates support three elongate, moderately 

 large interambulacrals, which are followed by six to eight .smaller ones. First 

 anal higher than the brachials; supporting on its sloping upper sides two 

 rather large interbrachials, and on the middle face an elongate anal piece ; 

 the next row generally consists of four plates, which rise to the height of 

 the arm bases. Ventral disk depressed pyramidal in its anterior aspect, the 

 posterior side greatly protruding outward and upward, and formed into a 

 large anal process, which rises beyond the top of the posterior oral, and from 

 4 to 6 mm. above the plane of the ventral di.sk. It is narrower at the base 

 than at the upper end, and somewhat depressed around the anus, which 

 opens obliquely upwards. Orals and radial dome plates large and tuber- 

 culous ; the former in contact laterally ; the latter separated from one an- 

 other, and from the orals, by small perisomic plates. Column small for the 

 size of the species. 



Horizon and Locality. — Uppermost part of the Upper Burlington lime- 

 stone ; Burlington, Augusta, and Pleasant Grove, Iowa. 



Remarhs. — This species is readily distinguished from A. americanus by 

 the very different size, form, and arrangement of its interbrachial plates, 



