712 THE CEINOIDEA CAMERATA OF NORTH AMERICA. 



Platycrinus eminulus Hall. 

 Plate LXVIII. Figs. 13a, h, c. 



1861. Hall; Descr. New Spec, of Crinoids, p. 17. 



A smaller species than P. rer/ahs ; the clor.sal cup more spreading, and 

 more distinctly pentangular ; the base proportionally smaller and flatter, 

 either smooth or marked by obscure concentric bands parallel to the mar- 

 gins of the plates, sometimes separating into distinct lachrymose nodes. 

 Basi-radial and interradial sutures deeply grooved, and the margins of ad- 

 jacent plates thickened and highly beveled. 



Basals forming a shallow basin, of which the median portions are rather 

 deeply depressed, and only the beveled outer margin is visible from a side 

 view ; interbasal sutures rather distinct. Eadials forming an angle of about 

 45° with the bottom of the cup; once and a half — or less — as wide as 

 long, and not quite one half wider at the top than at the bottom. The 

 notches toward the regular interradial spaces shallow, that at the anal side 

 considerably deeper ; the median portions strongly thickened to form the 

 facet, which occupies about one third the width of the upper face. The 

 facet is semi-ovate, deeply notched at the upper end, the surface perfect!}' 

 flat and directed horizontally. Costals almost as long as wide, wider at the 

 top than at the bottom, the upper angle obtuse. Distichals, palmars, and 

 post-palmars, two to each order ; the plates wider than long, gradually 

 decreasing in size; but the upper one of each order larger than the lower, 

 wider and also higher. Arms eight to the ray, given off as in the preced- 

 ing species ; quite heavy, very little tapering, and curving outward. Ventral 

 disk of about the same height as the dorsal cup ; composed of a few large 

 plates. The orals sharply tumid ; the posterior one resting between the 

 other four, somewhat larger, and provided with a stronger node. The 

 ambulacral plates large, but smaller than the orals, arranged quite regularly 

 in two rows. The interambulacral spaces depressed, formed of three trans- 

 versely arranged pieces, of which the middle one is as large as the orals, but 

 perfectly flat ; that on the anal side larger and curving longitudinally ; the 

 opening located in the middle of a small, rounded protuberance, composed 

 of small pieces. 



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



