528 GEOLOGICAL SURVEY OF IOWA. 



first radial plates which more nearly correspond with the description of P. corruga- 

 tus given by OWEN and SHUMARD; but in our specimens they differ not only in other 

 respects, but they are much more contracted at the summit. 



Fig. 1 a. The calyx in profile. 



Fig. 1 b. View of the base, natural size. 



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

 lington, Iowa. 



Platycrinus pocilliformis (n. s.). 



PLATE VIII. FIG. 2 a, b. 



GENERAL form of the cup elongate hemispheric ; greatest 

 diameter a little above the centre of the first radial plates, 

 the base being somewhat flattened. Basal plates large, une- 

 qual in size, the entire base equal to the diameter of the 

 inside of the cup at the summit of the first radial plates ; 

 central area very slightly depressed ( with a small perforation 

 in the middle ), curving upwards towards the edges, the up- 

 per margins nearly straight. First radial plates inserted 

 almost vertically upon the basal plates, a little wider in the 

 middle than at the base ; upper margins scarcely excavated ; 

 articulating scar small, semioval, and margined by a mode- 

 rately thickened rim. 



SURFACE marked by prominent rounded nodes or short 

 spines, which, on the basal plates, are arranged in double or 

 triple rows, and a line of three extending from the margin 

 of the central area to each angle of the plate : on the radial 

 plate, a row of three or four extend from the outer angles 

 inwards towards the base of the second radial ; and within 

 this space, three or four other tubercles, and outside the 

 converging lines are two or three on the lateral margins of 

 the plates. 



This species differs from P. cavus in the proportional size of the base, which is 

 much larger; the cicatrix for attachment of second radial or arm-plates is far less 

 prominent, and the character of tubercles is very distinct. It may be compared with 

 P. granulatus of MILLER ; but the tubercles are less numerous, and the cup less ex- 

 panded above. Our species more nearly resembles P. americanus of OWEN and SHU- 



