8 



the narrow succeeding plates, which are quadrangular and narrowly 

 rounded on their external surfaces. The fifth brachial is angular 

 above, a little wider than those below, and gives support on its 

 upper sloping sides to the first divisions of the ray, which thence 

 continue simple to their extremeties. 



The four other radials have each a single brachial, which is as 

 wide as the radial on which it rests, with a gaping suture between, 

 all pentangular, and supporting on their sloping upper angles the 

 first divisions of the rays. 



The left antero-lateral ray, after its first bifurcation on the bra- 

 chial plate, divides again on the sixth plate above, and the outer 

 division twice more on the seventh or eighth plate, while in the right 

 antero-lateral, both divisions bifurcate on the sixth phite, and the 

 outer division again on the seventh or eighth plate, making five di- 

 visions to this ray, which is probably the normal number in all the 

 rays except the anterior one, or twenty-two to the complete organ- 

 ism. The jirms are composed of rounded, wedge-formed joints, 

 giving off strong pinnules alternately from their longest sides — Pin- 

 nules composed of rounded joints about twice as long as wide. 



Anal plates, five visible. The first one is pentagonal, and rests 

 between the upper angles of two of the subradials, and partly under 

 the lower angle of the right posterior radial. The second anal is 

 larger than the first, hexagonal, and rests upon one of the sub- 

 radials, and extends above the top of the left posterior radial. The 

 third anal is hexagonal, about as large as the first, on which it rests, 

 and it extends nearly half its length above the top of the posterior 

 radial on the right. The fourth and fifth anals are smaller, the one 

 resting on top of the second anal and the other on the third. Column 

 unknown. 



Position and locality : Top of the geodiferous shales of the Keo- 

 kuk group, one mile below Keokuk, Iowa. 



Nos. 184 and 243 of Mr. L. A. Cox's collection. 



I take pleasure in dedicating this beautiful species to my worthy 

 friend and co-laborer Mr. Orestes St. John. 



POTERIOCEINUS LATIDACTYLUS. N. SP. 



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 mamillaiform, 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 j^osterior one larger than those on the anterior 

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

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

 reception of the Ijrachials. 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, 



