INVERTEBBATES. 275 



POTEKIOCRINUS LATIDACTYLU8, Worthen. 



PL XXVIII, Fig. 6. 



PotcriocriniiS latidactylus, WOKTHEN, February, 1882. 



Bulletin No. 1, of the Illinois State Museum of Natural History, p. 8. 



This interesting species is only represented by a single imperfect 

 specimen, showing the posterior side of the body, and a part of three 

 of the rays. 



Body mamillaeform, once and a half as wide as high to the top 

 of the first radials, and composed of smooth, closely joined plates. 

 Basals unknown. Subradials nearly as large as the radials, length 

 and breadth about equal, four of them hexagonal counting three 

 angles below, the left posterior one larger than those on the anterior 

 side and heptagonal. Eadials pentagonal, once and a half as wide 

 as high, and truncated straight across their upper margins for the 

 reception of the brachials. These are of the same form and a little 

 larger than the radials, and support on their upper sloping sides the 

 first divisions of the rays. The arms are composed of broad, short, 

 quadrangular plates, resembling closely the arms of Woodocrinus 

 and Bursacrinus. In two of the rays which are partly preserved in 

 the specimen before me, a second bifurcation takes place on the 

 fourth plate above the brachials, beyond which the arm structure is 

 unknown. The first arm-plates in the two posterior rays are about 

 twice as long as the succeeding ones. 



Three anal plates are visible ; the first one is larger than the 

 others, and rests between the upper angles of the two posterior sub- 

 radials. The second rests on top of the left posterior subradial, and 

 the third, which is smaller than either of the others, rests upon the 

 first, and above this the lower margins of two or more succeeding 

 plates can be seen. 



Column stout, covering the entire width of the base, and com- 

 posed at first of nearly equal joints that decrease in diameter below, 

 and pass into a series of alternately thick and thinner joints as the 

 distance from the base increases. 



Position and locality; Upper part of the Keokuk limestone, Hamil- 

 ton, Illinois. 



No. 401 of Mr. L. A. Cox's collection. 



