540 GEOLOGICAL SUEVEY OF IOWA. 



Radial plates one-fourth wider than long, slightly concave 

 on their upper margins, with a small short semioval articu- 

 lating scar which is margined by a moderately thick callo- 

 sity ; margins abruptly bevelled, and sutures canaliculate. 



SURFACE marked by strong prominent elongated nodes 

 which radiate to the margins of the basal plates, and con- 

 verge on the radials towards the base of the arms, below 

 which there is a plain space. 



This is a very well marked species in its truncate base with canaliculate sutures 

 and elongate nodose markings, differing very essentially from P. truncatus, which 

 resembles it only in the character of its sutures. 



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

 lington, Iowa. 



Platycrinus nucleiformis ( n. s.). 



BODY nucleiform ; base gibbous, uniformly convex except 

 the central small striated depression for the attachment of 

 the column ; sutures plane. Radial plates wider than long, 

 thickened and gibbous in the middle and curving towards 

 the lateral and basal margins ; upper margin truncate, with 

 a small notch in the centre ; the articulating scar semiellip- 

 tical, oblique, strongly marked and extending about one-third 

 the length of the plate, and occupying half its width. In- 

 terradials hexagonal, small : anal plate large, heptagonal; 

 anal opening somewhat proboscidiform, composed of nume- 

 rous small plates. 



SURFACE marked by fine, delicate granulations. 



The figure illustrates the form and proportions of the plates 

 of the body, the quadrangular subbrachial plate and the 

 small articulating scar on the upper margins of the radial 

 plates, which are distinguishing features of this species. 



