| 686 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
the interradial plates erect. Ventral disk convex; covered with spinous 
plates. Column small, obscurely elliptical. 
Horizon and Locality. —Chouteau limestone; six miles southeast of 
Sedalia, Mo. 
Types in the collection of Mr. Sampson. 
AMERICANUS GROUP. 
Dorsal cup rather short and but slightly spreading; base nearly flat; 
plates ornamented by coarse granules or rows of confluent nodes. 
Platycrinus americanus O and Su. 
Plate LXXV. Figs. 10, 11, 12, 13a, }, e. 
Owen and SHumarp; U. S. Geol. Surv. Wisc., Iowa and Minn., p. 594, Plate 5B, Figs. la, 6. 
W. and Sp.; Revision, Part II., p. 70 (Proceed. Acad. Nat. Sci. Phila, p. 244). 
Syn. Platycr. truncatus Haru; Geol. Rep. Iowa, Vol. L., Part IL., p. 537. 
(?) Syn. P. Broadheadi 8. A. Mittur ; 1891, Geol. Surv. Missouri, Bull..4, p. 21, Plate 2, Fig. 15. 
Calyx subglobose, a little higher than wide. Dorsal cup more than once 
and a half as wide as high, slightly spreading, the base broadly truncated. 
Height of ventral disk about equal to that of the dorsal cup. Plates orna- 
mented by coarse granules or irregular nodes, arranged in concentric lines 
around their margins, covering the entire surface. Edges of the plates 
beveled, and the basi-radial and interradial suture lines channeled. 
Basals forming almost a plane, rarely more than their beveled edges 
visible in a side view; the column facet very slightly depressed ; interbasal 
sutures indeterminable. Radials a little spreading, one fourth wider than 
long; the outer ends of the upper faces moderately sloping, except toward 
the anal side where they form a deep and broad notch. Facet semicircular ; 
the notch at the summit very small, if represented at all. Costals rather 
large, much wider than long; pentangular. Distichals and palmars as long 
as the costals but narrower, a little constricted across the middle. Arms six 
to the ray; of moderate size, biserial above the second or third plate from 
the bifurcation; the joints rather long; the pinnule sockets projecting, 
especially in young specimens, and the pinnules strong and in contact later- 
ally; the proximal one being given off from the first distichals. Ventral 
disk hemispheric, the plates convex. Orals large, rather regularly arranged. 
Ambulacral plates small and not elevated. Interambulacrals three and two; 
the middle one of the first row longer than wide, that of the anal side much 
