PLATYCEINID^. 671 



Platyerinus PrattenI Wobthen. 

 Flate LXX. Figs. 11, 12. 



1S60. 'WoKTHEN ; Trans. Acad. Sci. St, Louis, Vol. I., p. 569. 



1881. W. and Sp. ; Revision, Part II., p. 74 (Proceed. Acad. Nat. Sci. Phila., p. 248). 



&JI1. P. plmms (in part) — Owen and Shumakd; U. S. Geol. Rep. of Minn. loTva and Wise, p. 



5S7, Plate iA, Fig. ib (not 4«). Also Meek and Wokthen, 1866; Geol. Rep. Illiaois, Vol. 



III., p. 467, Plate 16, Kg. 6. 



A large species of the type of P. ^olaims. Calyx elongate, very large in 

 proportiou to the length of the arms. Dorsal cup once and a half as high as 

 wide, almost cylindrical along the radials, but rapidly spreading at the basals, 

 which form a moderately deep pentangular basin with its upper angles 

 slightly curving upwards ; the lower margins projecting outward and form- 

 ing a conspicuous rim around the edge, which often is twice as wide as the 

 column. Plates quite thin and perfectly smooth ; the radials slightly thick- 

 ened along the median line. 



Height of basal cup almost one half the length of the radials, its upper 

 edges very thin, and overlapping the lower edges of the radials ; interbasal 

 sutures obsolete. Radials about once and a half as long as wide ; the sides 

 parallel or very slightly constricted at the top and bottom ; the lower face 

 convex, the upper angles scarcely truncated. The lower faces of the radials 

 and the upper faces of the basals in some specimens contain tooth-like pro- 

 jections fitting into each other like interlocking sutures. Radial facets semi- 

 circular, occupying nearly half the width of the plates, and filled completely 

 by the costals, which are subtrigonal, with concave upper faces. Arms from 

 ten to twelve to the ray, comparatively short, rather thin, and very little 

 tapering. The arm plates moderately short and slightly waving. Pinnules 

 long and in close contact laterally. Of the ventral disk only the middle 

 plate of the first interradial row has been observed, which is comparatively 

 small and subtrigonal. The two proximal joints of the column circular. 



Horison and Locality. — Lower Burlington limestone; Burlington, Iowa. 



Ty]^e in the Illinois State collection, Springfield. 



Remarks. — This species was confounded by Owen and Shumard, and 

 also by Meek, with P. planus, from which it differs in the larger size, the 

 proportionally much smaller and thinner arms, the projecting rim of the 

 basal cup, and in the undulated basi-radial suture lines. 



