PALEONTOLOGY OF IOWA. 533 



Fig. 5. Lateral view, showing the cup with fragment of column, the edges of the concave 

 base, the radial plates and arms which are somewhat broken and distorted. 



FIG. 58. 



The accompanying diagram illustrates the form of the 

 basal and radial plates and the mode of bifurcation of the 

 arms. 



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

 lington, Iowa. 



Platycrinus planus ? 



PLATE VIII. FIG. 6 a, b. 



Compare Platycrinus planut, OWEN and SHUMARD, Report of the Geology of Wisconsin, Iowa 



and Minnesota, pa. 587, pi. V A, f. 4. 



CALYX elongate, cylindrical above and subturbinate below ; 

 base cupform, pentagonal with the angles prominent and the 

 intermediate spaces concave ; area for the attachment of the 

 column abruptly depressed, striate : surface of base marked 

 by three distinct shallow grooves, which indicate the divi- 

 sions of the basal plates, and along which the suture is 

 sometimes distinctly marked. Radial plates large, heptagonal; 

 width and height about as ten to thirteen ; the cicatrix for 

 attachment of arms strongly marked, and the upper edge of 

 plate slightly concave. Interradial plates hexagonal, some- 

 what irregular in form. Summit rounded above, ventricose, 

 composed of numerous plates of unequal size. Arms double 

 at their origin, and bifurcating to six or eight divisions from 

 each radial. 



SURFACE finely granulose ; the basal and radial plates 

 marked by numerous concentric undulations. 



