PALAEONTOLOGY OF IOWA. 629 



MARK; but it is larger, with proportionally larger basal plates and a wider and less 

 depressed central area, while the tubercles are much more distant and more elevated 

 than in that species, which is likewise wider at the summit of the first radials than 

 at any point below. 



In the engraved figures, the radiating lines of tubercles are not given with sufficient 

 force and precision. 



Fig. 2 a. View of the base. 



Fig. 2 b. Profile, showing elevation, form of first radial plates, and cicatrix for 

 the attachment of the second radial. 



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

 lington, Iowa. 



Platycriims pileiformis (n. s.). 



PLATE VIII. FIG. 3 a, b, c. 



GENERAL form of the body cup-shaped, or broadly subtur- 

 binate, gradually spreading above. Basal plates unequal, very 

 convex exteriorly, and curving upwards at the margins, 

 which are concave for the reception of the succeeding 

 plates ; length about equal to two-thirds the length of the 

 first radial plate : column area small, depressed, strongly 

 striated, with a small central perforation. First radial plates 

 gradually expanding above, slightly curved on the base, a 

 little wider above than below (in a well preserved specimen, 

 two are of the same width at base and summit, and three 

 are wider above). Summit of plates slightly concave, the 

 articulating surface semioval, deep, well defined ; the mar- 

 gin of the cicatrix limited by a sharp salient rim, which 

 extends downwards from the top scarcely one-fourth of the 

 length of the plate. A broad undefined depression is traceable 

 along the basal and radial plates a little distance within the 



margin. [ The line representing this, in the engraved figure, is too strongly defined.] 



Second radial or subbrachial plates small triangular, the 

 arm-plates of the first pair resting on its upper sloping sides, 

 and upon the articulating surface of the first radial; arm- 

 plates small ; bifurcations unknown beyond the first division. 



SURFACE smooth or finely granulose, without ornament : 

 sutures plain. 



[ IOWA SURVEY.] 67 



