PALAEONTOLOGY OF IOWA. 539 



Platycriiius exsertus ( n. s.). 



BODY small, ovoid, robust, the plates exceedingly thick- 

 ened ; base pentagonal, truncate below with a small depres- 

 sion for the reception of the column ; sutures marked by a 

 shallow depression ; margins extremely thickened, giving it 

 the appearance of protruding beyond the radial plates. Radial 

 plates as wide as long, very gibbous, with a broad articula- 

 ting scar above, occupying two-thirds the width and one- 

 third the length of the plate : upper margins truncate with 

 a narrow notch in the centre ; sutures marked by a broad 

 deep groove. Interradial plates hexagonal : arms originating 

 in double series from a small triangular subbrachial, which 

 does not fill the entire depression in the radial plate. 



SURFACE concentrically granulose striate. 



This species is well characterized by its extremely thickened plates, and the pro- 

 truding base which appears as if having been pressed outwards, and is quite unlike 

 any other species of Platycrinus which has fallen under my observation. 



FIG. 01. 



The accompanying figure illustrates the form and propor- 

 tions of the plates of the body and arms, as far as they 

 exist in the specimen examined. 



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

 lington, Iowa. 



Platycrinus caDaliculatus ( n. s.). 



BODY small. Calyx depressed cup-shaped : base truncate, 

 depressed in the centre for the reception of a moderately 

 large column ; line of suture marked by a distinct groove, and 

 a corresponding less deep groove extending to the two remain- 

 ing sides of the basal plates ; edges of the plates projecting. 



