INVERTEBEATES. 283 



POTERIOCRINUS OiTERENSis, Worthen. 



PI. XXVIII, Fig. 4. 



Potenocrinus Otterensis, WOKTHEN, February, 1882. 



Bulletin No. 1, of the Illinois State Museum of Natural History, p. 14. 



Body above the medium size, basin-shaped, about once and a 

 half as wide as high, base depressed and the basal plates hidden 

 by the column, which is quite stout and pentalobate where it joins 

 the body. 



Subradials about one-fourth wider than long, the lower angle 

 curving inward to form a part of the basal concavity. The center 

 of these plates is smooth and a little protuberant, with four deep 

 indentations on their borders, two of which are lateral, to meet 

 those of the adjacent subradials, and two directed obliquely upward 

 to meet similar depressions in the radial plates. 



Radials wider than long, constricted across the middle, and pro- 

 jecting on their upper margins, with two slightly oblique depres- 

 sions below, that meet two of those in the subradial plates. They 

 are pentagonal in form and support on their upper truncated mar- 

 gins the brachial series. 



Brachials one to each of the postero-lateral rays, smaller than 

 the radials, slightly constricted, and sharply angular above, giving 

 support to the first divisions of the rays. Arm structure unknown. 



Three anal plates are visible, the first nearly as large as the sub- 

 radials, the others a little smaller, and all having from four to six 

 depressions around their borders that correspond to similar depres- 

 sions on adjacent plates. The anal plates have been shoved out of 

 their true position by the crushing of the specimen, so that, their 

 relation to the adjacent plates cannot be clearly determined. 



Position and locality: This unique form was obtained from the 

 Keokuk limestone, on Otter Creek, Jersey county, 111. 



No. 2,466, Illinois State collection. 



