674 THE CEIiS'OIDEA CAMEEATA OP NORTH AMERICA. 



Platycrinus sequalis Hall. 

 Plate LXXI. Figs. IfU, h, and 5. 



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



1873. Meek, and Worthen ; Geol. Rep. Illinois, Vol. V., p. 456, Plate 3, Fig. 8. 



1881. W. and Sp. ; Revision, Part 11., p. 70 (Proceed. Acad. Nat. Sci., Pliila., p. 244). 



Sjn. P. batiola S. A. Millee ; Geol. Surv. Missouri, Bull. 4, p. 22, Plate 3, Figs. 1 and 2. 



Calyx of medium size, the arms proportionally long. Dorsal cup bell- 

 shaped, rounding below to near the column facet, which is distinctly elliptic 

 and somewhat protuberant, giving to the sides of the base just above a 

 slightly concave outline ; the radials a little convex, and more or less 

 spreading to the facets, which are surrounded by a thickened rim. Plates 

 a little thicker than in the preceding species, moderately convex, and with- 

 out ornamentation. The basi-radial and interbasal suture lines generally 

 forming broad, shallow depressions. 



Basal cup deep, its height about two thirds the length of the radials, 

 somewhat quinquelobate as seen from below, a little bulging in a radial 

 direction, depressed interradially ; the upper margin slightly beveled, pro- 

 ducing a moderate constriction along the suture line. Eadials quite thin at 

 their edges, thickened in the middle, a little longer than wide, and usually 

 wider at the upper end than at the lower ; the superior angles slightly trun- 

 cated. The facets subquadrangular, deep, rather long, their width equal to 

 one half the transverse dimension of the plates. Costals trigonal, very small, 

 occupying but one third the width of the facets, which enclose one or both 

 distichals. Distichals and palmars twice as wide as long, free above their 

 first plates. Arms varying from six to ten to the ray ; long, widest at the 

 middle, gradually tapering to the tips, the dorsal surface somewhat flattened. 

 The three or four proximal arm pieces cuneate and singly arranged, the 

 biserial ones above shorter and presenting a curious flexure or genicula- 

 tion in the middle, so as to give a zigzag appearance to the transverse 

 sutures between them. Pinnules in close contact, and composed of joints 

 three to four times as long as wide. Ventral disk unknown. Column 

 elliptic, the long diameter of the joints more than twice as great as the 

 shorter one. 



Horizon and Locality. — Upper Burlington limestone, Pleasant Grove, and 

 Burlington, Iowa. 



