Carboniferous Rocks of the Mississippi Valley. 287 



nearly twice as wide as high, broadly truncate at base. 

 Basal plates proportionally large, thickened in the middle, 

 and extending so as to form a projecting rim to the base 

 of the cup ; this rim is ornamented on its edge by several 

 short sub-spines. Radial plates short, broad, sub-quad- 

 rangular ; the articulating facet for the attachment of tlie 

 sub-brachial plate is shallow, occupying little more than 

 half the width of the plate, and entirely occupied by the 

 short triangular plate ; this supports on each side the first 

 arm plates. 



Arms bifurcating on the second plate above the sub- 

 brachial, the inner division again bifurcating on the sec- 

 ond or third plate above the first bifurcation ; arms 

 composed near the base of a single series of short plates, 

 becoming double above, where they are obtusely wedge- 

 form at the interlocking edges. Surface of radial plates 

 marked by three rows of small nodes, rising from the 

 lower margin of the articulating scar, and passing one to 

 each lower lateral angle and one to the basal margin, unit- 

 ing with the nodes of the basal plates. 



This small species differs from any other in these rocks 

 in its low calyx, peculiar surface markings, and nodose 

 basal rim formed by the projection of the basal plates. 



Geological formation and locality. Keokuk limestone, 

 Keokuk, Iowa. 



Platycrinus striobrachiattis (n.s.). Body discoid to the 

 arm bases ; dome highly elevated, hemispherical. Basal 

 plates occupying little more than one third the width of the 

 disc, depressed in the centre around the top of the column. 

 First radial plates expanding laterally from the base to 

 near the middle, where they are abruptly bent upwards, 

 the upper part forming the sides of the cup, broadly and 

 deeply notched above, truncated on the upper lateral 

 angles by the large interradial and anal plates. The 

 articulating scar occupies about one half the width of the 



