668 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
PLANUS GROUP. 
Calyx elongate ; the plates thin and without ornamentation; upper faces 
of the radials straight, or very little sloping toward the angles. 
Platycrinus planus O. and Su. 
Plate LXIX. Figs. 2a, 6, c, d. 
1850. Owen and Suumarp; Journ. Acad. Nat. Sci. Phila. (new series), Vol. II., Parts I. and IL, p: o75 
also U. 8. Geol. Surv. Minn., Iowa, and Wisc., p. 587, Plate 5.4, Fig. 4a (not 4b = P. 
Pratteni). 
1831. W. and Sp.; Revision, Part II., p. 74 (Proceed. Acad. Nat. Sci. Phila., p. 248). (not P. planus 
Hatt, Geol. Rep. Iowa, Vol. I., Part II., Plate 8, Figs. 6a,)—= P. Halli; nor P. planus, Menx 
and WortHEN, Geol. Rep. Illinois, Vol. III., Plate 16, Fig. 6 = P. Pratteni). 
Syn. Platyerinus bloomfieldensis 8. A. Minumr, 1879; Journ. Cincin. Soc. Nat. Hist., Vol. II., Plate 
15, Fig. 4. 
A moderately large species. Calyx elongate. Dorsal cup goblet-shaped, 
higher than wide in large specimens, height and width about equal in 
smaller ones; basal cup deep, obconical, slightly truncate at the bottom; 
radials a very little spreading to the facets, the latter projecting so as to 
give to the cup, as seen from above, a slightly pentagonal outline. Plates 
thin and perfectly smooth; the suture lines rather indistinct. 
Basal cup large, obconical, its height equal to two fifths the height of the 
dorsal cup; the lower face slightly truncated but not excavated, obscurely 
elliptic, and covered completely by the column. Radials longer than wide, 
the sides almost parallel; the limbs somewhat inflected and knife-like at 
their superior edges, where they form almost a straight line with those of 
adjoining plates ; the median portions of the plates slightly thickened, thin- 
ning out gradually toward the sutures. Width of the facets one third the 
diameter of the plates; horse-shoe-shaped, deeply excavated, and facing up- 
wards. Costals short, trigonal, occupying the whole width of the facets, 
their sloping upper faces concave. Distichals twice as wide as long, and 
as long as the costals; the lower ones of the same ray in sutural contact. 
Palmars and post-palmars three fourths as long as wide, their lower plates 
connected by suture. Arms four to each subdivision; or eight to the ray ; 
very long, rounded on the back, and but very slightly decreasing in width. 
The arm joints moderately short, the intervening suture lines distinctly 
waving. Pinnules long and in close contact laterally. The ventral disk, 
so far as observed from the fragments preserved, extends up almost vertically 
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