594 THE CEINOIDEA CAMERATA OF NORTH ABIERICA. 



from one third the height of the calyx in large specimens to scarcely one 

 fourth in smaller ones. Plates of the cup convex and of rather uniform size ; 

 their surface ornamentation exceedingly variable. In most specimens, the 

 middle part is bare of all markings, but along the margins of the plates there 

 are at each side from one to three elongate processes or short ridges, and 

 between them along the suture lines deep pits, which almost penetrate the 

 test. In other specimens there are continuous ridges covering the whole 

 surface, which give to the plates a reticulate or cancellate appearance ; while 

 in still others rows of bead-like elevations take the place of the ridges. 



Basals forming a low, cylindrical cup, which is somewhat grooved at the 

 sutures. Radials and costals of nearly the same size, and all as long as wide ; the 

 first costal a little smaller and hexangular, the second heptangular. Distichals 

 1 X 10, about half the size of the axillary costals, and all axillary ; the higher 

 orders of brachials much smaller, and rounded like arm plates; the bifurcation 

 being alternately from every successive plate. There are generally two or 

 more orders above the palmars, sometimes three, and in young specimens occa- 

 sionally but one ; the number of arms, therefore, varies from four to six in 

 the main branches, and from eight to twelve in the ray. Arms given off alter- 

 nately from opposite sides, and the proximal one free above the second plate, 

 the others above the first ; they are below medium size, long, and rounded 

 on the back. Pinnules covered with small hooks. Regular interbrachials from 

 eight to ten, in six or seven ranges. Anal plate as large as the radials; 

 followed by 2, 3, 3, 3, 4 and two plates. Interdistichals two or three. Inter- 

 brachials as well as the interdistichals in contact with the plates of the disk. 

 Ventral disk deeply plicated around the margin ; the plates small, generally 

 smooth, and of nearly the same size and form. Column of nearly uniform 

 size to about 6 cm. from the distal end, where it gives off strong branches 

 at irregular intervals, and tapers gradually to a sharp point. The nodal 

 joints are a little the longest and widest near the calyx, but become indeter- 

 minable farther down ; axial canal of moderate size. 



Horizon and Locality. — Upper and Lower Buriington limestone ; Burling- 

 ton, Iowa, and at several localities in Missouri. 



Type in the (Worthen) Illinois State collection, Springfield. 



Eemarks. — This species has been divided up into several varieties, based 

 upon certain variations in the ornamentation of the plates, which cannot be 

 recognized in large collections ; and McChesney described a young specimen 

 as a new species. As a rule, in the smaller specimens the tegmen is more 



