538 



GEOLOGICAL SURVEY OF IOWA. 



Fio.59. 



The accompanying diagram illustrates the form and 

 proportions of the plates of the body and lower part 

 of the arms. 



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

 lington, iowa. 



Platycrinus truncatulus ( n. s.). 



BODY small, basin-shaped, somewhat hemispheric ; base 

 truncate, the truncation extending nearly to the margin of 

 the basal plates, a slight depression in the centre for the 

 attachment of the column. Radial plates wider than long, 

 regularly convex, gradually spreading from the base, slightly 

 concave on the upper side, the articulating scar being on the 

 upper margin : subbrachial or first arm-plate quadrangular ; 

 sutures plain, or with a callosity along the radial sutures. 



SURFACE finely granulate, without nodes or inequalities. 



This species differs from the preceding, which it resembles in general form, in the 

 truncation of the base which does not extend to the margins of the basal plates, and 

 in the absence of grooves along the suture lines. The articulating face for the arm 

 attachments is mainly on the upper edge, and does not occupy the side of the plate 

 as in P. truncatus; while the first plate above the radial is of quadrangular form with 

 a single groove on the 'inside, showing that there was no bifurcation from this plate. 



FIG. GO. 



a 



The figure illustrates the form and proportions of the basal 

 and first radial plates, and shows the quadrangular first arm-joint 

 attached to one of the rays. 



Geological formation and locality. In the Burlington limestone 

 lington, Iowa. 



Bur- 



