PALEONTOLOGY OF IOWA. 537 



SURFACE marked by transversely elongated nodes or lamel- 

 lose ridges, arranged principally in lines parallel to the upper 

 margins of the plates, with a radiating line of nodes ex- 

 tending from the base to each angle of the basal plates. 



Superior plates unknown. 



The form of the basal portion of this fossil is so unlike any of the granulose or 

 ornamented forms here described, or any of those described by Drs. OWEN and 

 SHTJMABD, that it will be easily distinguished. The basal portion resembles P.planus 

 in form, but is less convex or gibbous in contour, while the surface ornamentation 

 is entirely distinct from any other species noticed. 



Fig. 11. Basal view, showing the depressed area for attachment of column, form of plates 

 and surface ornaments. [The nodes are often more confluent than shown in 

 the figure.] 



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

 lington, Iowa. 



Platycrinus truncatus (n.s.). 



BODY small. Calyx broadly cup-shaped, flat in the bottom : 

 externally the base is truncate, the truncation extending to the 

 margins of the basal plates. Radial plates one-fourth wider 

 than long, scarcely spreading above, notched in the middle of 

 the upper edge, the articulating scar being semicircular, with 

 a thickened elevated margin, which extends about one-third 

 the length of the plate downwards, and below which is 

 a depression of the surface. Sutures between radial plates, 

 and between these and the base, abruptly canaliculate. In- 

 terradial plates small, hexagonal. Subbrachial plates compa- 

 ratively large, pentagonal, supporting on the upper sloping 

 sides two arm-plates, which dichotomize again on the second 

 or third plate above. 



SURFACE marked by simple fine granulations, so far as can 

 be observed. 



Several specimens of this species have been examined, and they are all small and 

 of nearly uniform size. 



[ IOWA SURVEY.] 68 



