530 GEOLOGICAL SURVEY OF IOWA. 



This species resembles P.planus of OWEN and SHUMARD; but differs in the form 

 of the base, and in the absence of raised lines on the exterior or depressions on 

 the interior. The first radial plates are proportionally shorter, and the excavations 

 for the insertion of the arms are much smaller and more shallow. Our species cor- 

 responds more nearl j with P. pileatus of GOLDFUSS, as figured by DE KONINCK and 

 LE Hew (" Crinoides du Terrain carlonifere de la JBelgique ", pl.vi, f. 3); but is 

 less expanded above, and the basal plates are proportionally longer, while the articu- 

 lating surface for attachment of the .arms is smaller. 



Fig. 3 a. View showing the elevation of the basal plates, the first radial plates, with the 

 base of one pair of arms attached. 



Fig. 3 b. Basal view of the same specimen : the two lower radial plates of the figure, 

 and the radial on the left side, are represented as too protuberant. 



FIG. 56. 



The accompanying diagram exhibits the form and 

 proportions of the plates, the small subbrachial plate 

 and first arm-plates, the outer side of which, for half 

 their length, rest upon the articulating surface of the 

 first radial plate. 



Geological formation and locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



Platycrinus wortheni ( n. s.). 



PLATE VIII. FIG. 4. 



CALYX broad cup-shaped, and truncate at the base ; the 

 arms spreading somewhat abruptly from the bases to the 

 second bifurcation, and thence continuing vertically. Base 

 pentagonal, sutures not visible, concave from near the outer 

 margin which is slightly convex. Radial plates about as long 

 as wide, truncate at the base, with the sides gradually ex- 

 panding above, and having a general quadrangular aspect. 

 Articulating surface for the insertion of the arms broad and 

 deep, with a scarcely salient margin : sutures between the 

 radials, and between the radial and basal plates canaliculate. 

 Interradial plates narrow. Second radial or subbrachial plates 

 small, triangular, supporting two short wide arm-plates, each 



