802 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



often altogether hidden from view. Distichals and lower arm plates nearly 

 of equal size. The arms free from the second plate, and biserial from the 

 fourth ; they are unusually strong, rounded at the back, and composed of 

 two rows of short, transverse pieces. Pinnules slender and closely arranged. 

 The appendages are thin and knife-like at their upper edges, much thicker 

 below (Meek and Worthen) than above. Construction of the ventral disk 

 unknown. Column round, occupying about one fourth of the basal concavity. 



Horizon and Locality. — Kaskaskia group ; Hardin Co., Ills. 



Ty2:ie in the IlHnois State collection, Springfield. 



Pterotocrinus chesterensis Meek and Wobthen. 

 Plate LXXIX. Figs. 5a, I. 



I860. Dichocrims {Plewtocrims) chesterensis — Meek and Wokthen ; Proceed. Acad. Nat. Sci. Phila. 



p. 383. 

 13G6. Pterotocrinus chesterensis — Meek and Worthen; Geol. Rep. lUinois, Vol. II., p. 292, Plate 23, 



Figs, la, b, c. 

 18S1. Pterotocrinus chesterensis — W. and Sp. ; Revision Paleeocr., Part II., p. 91. 



The smallest known species of the genus. Sides of the dorsal cup dis- 

 tinctly concave, rapidly spreading upwards ; the base truncated, and its lower 

 margin projecting ; the plates without ornamentation, and the suture lines 

 not grooved. 



Basal cup about one third the width of the calyx at the arm bases, and 

 about three times as wide as high, truncated and concave below, the concav- 

 ity surrounded by an angular rim. Eadials not quite twice as wide as long, 

 widening rather rapidly from below, the superior faces a little excavated 

 except the outer ends, which are slightly truncated. Costals small, trian- 

 gular. Distichals iiTegularly pentagonal. Fixed palmars four; the two 

 outer ones in the same line with the distichals, and like them reposing upon 

 the radials. Anal piece subovoid, its broadest end about as wide as the 

 radials at their lower faces, the upper end angular. Arms biserial from the 

 third plate up, small, and rounded on the back. Structure of ventral disk 

 and form of the appendages unknown. 



Horizon and Locality. — Kaskaskia group ; Hardin Co., Ills. 



Type in the (Worthen) Illinois State collection, Springfield. 



