674 THE CRINOIDEA CAMERATA OF NORTH AMERICA. : 
4 
Platycrinus sequalis Hatt. | 
Plate LXXI. Figs. 4a, b, and &. 
1861. Hats Descr. of New Spec. of Crinoids, p. 17. 
1873. Mux and Wortuen; Geol. Rep. Llinois, Vol. V., p. 456, Plate 3, Fig. 8. 
1881. W. and Sp.; Revision, Part Il., p. 70 (Proceed. Acad. Nat. Sci., Phila., p. 944), 
Syn. P. datiola 8. A. Minter; 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. Radials 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 
 —— = 
ee TE A 
Pe 
wi | one half the transverse dimension of the plates. Costals trigonal, very small, | 
Al) i | : occupying but one third the width of the facets, which enclose one or both : 
2 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- 
sutures between them. Pinnules in close contact, and composed of joints 
H a | tion in the middle, so as to give a zigzag appearance to the transverse 
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. 
