758 THE CRINOIDEA CAMEEATA OF NORTH AMERICA. 



equal to, or greater than the length ; their upper faces one third narrower ; 

 facets semicircular, directed upward, and occupying one half the width of the 

 plates. Anal plate of a similar form to the radials, but narrower at the top, 

 the iipper portion somewhat bulging. Costals two, very short. Arms two 

 from eacli ray, ratlier strong, biserial from the fifth plate ; flattened on the 

 baclv. Arm joints twice as wide as long. Pinnules heavy and nnnsnally 

 long even for tliis genus. Column small ; the nodal joints considerably 

 widest, and their edges knife-like and serrated. 



Horizon and Locality. — Upper Burlington limestone ; Burlington, Iowa, 

 and several places in western Illinois and Missouri. 



Dichocrinus plicatus Hall. 

 Fkde LXXriI. Figs. 5a, h. 



1861. Hall; Boston Journ. Nat. Hist., Vol. VII,, p. 288. 



18S1. W. aud Sp. ; Revision Palasocr., Part II., p. 84 (Pioceetl. Acad. Nat. Sci. Phila., p. 258). 



Of the type of D. striatus, which it resembles in the style of ornamenta- 

 tion ; but the plates are thicker, the ridges upon the plates comparatively 

 larger, less in number, and the species is smaller tliroughout. Calyx globu- 

 lar, slightly conical at the lower end ; greatest diameter at the lower part of 

 the radials, the upper part curving gradually inward. The ribs or ridges 

 covering the surface are nndulated and very strong, the intervening grooves 

 deep and wider in the middle than at the ends. They form six well defined 

 rhombs, distributed around the calyx in a similar manner as tho.^e in the 

 preceding species; but their inner spaces are occupied by one or two in 

 place of three to four ridge.s, and the triangles at each side of the rhombs, 

 which are longitudinally divided by the interradial sutures, are smooth or 

 only occupied by low pustules. 



Basals longer than the radials, forming a deep cup with rounded sides 

 and slightly truncated lower end. Eadials subquadrangular, widest at the 

 lower margin ; facets wide, occupying two thirds the width of the plate, and 

 directed upward. Anal plate generally a little narrower than the radials. 

 Costals two, small, supporting two arms. Arms thin, long; composed of 

 rather long cuneate pieces, which .interlock from the fourth or fifth plate. 

 Pinnules stout and very long, the joints three times longer than wide. 

 Column small. 



Horizon and Localiii/. — Upper Burlington limestone ; Burlington, Iowa. 



Ti/pes in the Museum of Comparative Zoology. 



