PALAEONTOLOGY OF IOWA. 535 



Fig. 7 a. Anal and postero-lateral view, showing the incurved upper margins of the 

 radial plates, the hases of the arms, and the plates of the summit with the 

 anal opening. 



Fig. 7 b. View of the base of the specimen, showing the basal, radial, and first arm- 

 plates, with the interradial plates. 



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

 lington, Iowa. 



Platycrinus (li 



PLATE VIII. FIG. 8 a, b. 



Platycrinus discoideus : OWEN and SHUMAKD, Eeport on Geology of Wisconsin, Iowa and 

 Minnesota, pa. 581, pi. V A, f. 1. 



CALYX discoid, somewhat decahedral, shallow, height about 

 one-third the diameter ; base pentagonal, bevelled towards 

 the margin, with a broad and deep depression towards the 

 centre, the limit between the two upper slopes marked by 

 a subnodose or irregular ridge, with indentations along the 

 lines of the sutures. Radial plates massive, straight upon 

 their base, and gradually expanding above : lateral and ba- 

 sal margins of plates bevelled so that the line of suture is 

 marked by a broad shallow groove. Nearly one-third the 

 width of the radial plates lies almost in the same plane with 

 the basal plates, being bent abruptly upwards at the sides. 

 Articulating surface of the arms broad, semicircular, and 

 margined by a prominent, thickened ridge. 



SURFACE marked by rugose ridges which are more or less 

 parallel to the margins of the plate, and somewhat irregu- 

 larly disposed over the central portions. Column and arms 

 unknown. 



I have referred the specimens of the above type to this species of OWEN and SHU- 

 MAKD, though there are some slight differences between the description given and the 

 characters exhibited in our specimens. Separate plates, belonging apparently to this 

 species, are not uncommon, though the calyx is rarely found. In the specimen figured 

 the surface is imperfect. 



Fig. 8 a. Basal view of the specimen. 



Fig. 8 b. Profile view, showing the elevation of the side of the cup. 



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

 lington, Iowa. 



