FOSSILS OF THE CHESTER GROUP. 



ECHINODERMATA. 

 GENUS POTERIOCEimTES. 



POTERIOCRINITES BlSSELLI, Worthen Ms. 



PI. 21, Fig. 4. 



BODY of medium size, obconical, or tapering regularly 

 from the arm bases to its comieetion with the column, com- 

 posed of smooth, slightly rounded plates, with well denned 

 sutures. Base small, truncate below, gradually widening 

 upwards, and composed of nearly equal pentagonal pieces 

 about as wide as high; columnar facet with a slight mar- 

 ginal rim, and pentagonal in outline. Subradial , plates 

 nearly equal in size, the two on the anal side slightly 

 larger than the others and heptagonal, the other three hex- 

 agonal. First radials, four wider than high, and pentago- 

 nal, the right hand one on the anal side higher than wide, 

 and hexagonal, Anal pieces four or more, the first three 

 about half the size of the first radials, one pentagonal and 

 two hexagonal, the others smaller and hexagonal. Second 

 radials not well shown, but apparently about as high as 

 the first. Arms composed near their base of rather thick 

 wedge-shaped pieces, which gradually become flat and 

 thinner above. Column composed of rather thin segments 

 near the base, which gradually become alternately thicker 

 below. 



