INVERTEBRATES. 271 



radials, and this is succeeded by a double series of minute plates 

 that form the lower extremity of the ventral tube. This organ is 

 composed of minute plates, is balloon shaped, and shows a small 

 opening about two-thirds the distance from the base to the summit. 

 Column at the top composed of round, nearly uniform plates, but a 

 short distance below thicker joints are intercalated at irregular inter- 

 vals. It decreases in diameter nearly one-half in a distance of two 

 inches from the top, and has delicate cyrrhi attached on either side 

 at irregular intervals. 



The depressions at the angles of the body plates, give to the ex- 

 terior of the cup- shaped body a somewhat rough appearance. 



This species is named in honor of Mr. N. K. Burket, of Keokuk, 

 Iowa, who discovered the interesting locality at Hamilton where 

 this and several other new forms have been obtained. 



Position and locality: Top of the Keokuk limestone, Hamilton, 

 Illinois. 



Nos. 260 and 402 of Mr. L. A. Cox's collection. 



POTEBIOCKINUS TENUIDACTYLUS, 



PL XXVIII. Fig, 13. 



Poteriocrinus tenuidactylus, WORTHEN, February, 1882. 



Bulletin No. 1, of the Illinois State Museum of Natural History, p. 6. 



Body obconical, length and breadth to the top of the radial plates 

 about equal. Basals well developed, expanding upward from the 

 column, forming a pentagonal cup rather wider than long. Subra- 

 dials about as wide as high, the one on the posterior side supporting 

 in part two quadrangular anal plates, that are succeeded by a 

 double series of smaller plates of the same form, forming the base 

 of the ventral tube. 



Radials pentagonal, wider than high. Brachials four to each of 

 the posterior rays, three of them quadrangular and the fourth pent- 

 angular, supporting on its upper sloping sides the first divisions of 

 the rays. The arms in the left posterior ray, after dividing on the 

 last brachial, give off a branch from each division, about the twelfth 

 to the twentieth plate, beyond which they are simple as far as can 

 be seen, giving four arms to this ray. 



The arms are long and slender, composed of rather long, rounded 

 joints, slightly zigzag in their arrangement, and give off rounded 

 pinnules from their projecting sides. 



