PLATYCEINID^. 663 



their upper and lower faces parallel, the outer lateral faces provided with a 

 sharp projection placed in front of the pinnules. 



Ventral disk convex, gradually curving to the summit, quinquelobate as 

 seen from above, the plates slightly convex. Orals rather asymmetrically 

 arranged ; the posterior one larger than the other four, and pushed in be- 

 tween them. Ambulacral plates of the first order consisting of but few 

 large pieces of rather irregular arrangement, those of tlie second order 

 about one half smaller. Interambulacral plates of the regular sides five 

 in two rows ; the middle one of the first row large, elongate, hexangular, 

 much narrower at the top than at the bottom, and deeply wedged in between 

 the radials ; the two at the sides very narrow, the edges toward the ambu- 

 lacra deeply scalloped by adjoining covering-pieces. Anal side considerably 

 larger, the notch between the radials deeper ; the middle plate much shorter, 

 wider and pentangular, supporting upon its sloping upper faces two smaller 

 pieces, which, being at a higher level, form with their lower faces and with 

 the sloping lower sides of the middle plate a sharp angle ; the plates of the 

 first row are followed by numerous smaller ones, which form a large rounded 

 protuberance, containing the anus. Column moderately large ; the three 

 upper joints circulai', the lower ones distinctly elliptic. 



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



RemarJiS. — This species resembles P. pReiformis in the form of the calyx ; 

 but the plates are heavier, the arms comparatively shorter, not so heavy, and 

 much more numerous. 



Platycrinus aCClivUS S. A. Miller. 

 1891. S. A. MiLLEB ; Geol. Suit. Missouri, Bull. 4, p. 12, Plate 1, Figs. 9 and 10. 



According to Miller, the dorsal cup somewhat obconoidal, nearly one half 

 higher than wide, slightly " angular in the direction of the arms," sutures 

 distinct but not beveled, surface apparently smooth, but the specimen may 

 not have preserved the surface ornamentation. Basals form an obconic, sub- 

 pentagonal cup, a little wider than high, and truncated below. Eadials a 

 little longer than wide, very slightly expanding above, and terminating 

 in an upward, central prolongation for the reception of the costals. Facets 

 facing upward ; less than one third the width of the plates, subcircular in 

 outline, with the exception of a slight truncation at the inner side, and a 

 small concave ambulacra! furrow. Column round at the proximal end, " the 



