INVEETEBEATES. 281 



Arms composed of rounded, nearly quadrangular joints, and after 

 the first division on the second brachial they divide twice more in 

 the posterior rays, first on the seventh, and each division again on 

 the tenth plate, making eight arms to each of these rays. The 

 other rays are partially embedded in the matrix, minute projec- 

 tions are visible with a good glass on the outer side of the arm 

 plates, that give a roughened aspect to the whole specimen, and has 

 suggested the specific name, and will also serve to distinguish it 

 from nearly related species. 



Anal plates so closely anchylosed that their forms cannot be 

 clearly determined, but the first one rests between two of the sub- 

 radials, and is succeeded by a double series of smaller plates that 

 apparently go to form the base of the ventral tube. Traces of 

 coarse striae, or rows of granules, extending across the brachial 

 plates, as well as the radials, can be seen with a good glass. 



Column round, composed of rather even joints, with numerous 

 delicate cirrhi attached to it at short intervals. 



Position and locality: Upper part of the geodiferous shales of 

 the Keokuk group ; one mile below Keokuk. 



No. 234 of Mr. L. A. Cox's collection. 



POTERIOCBINUS ARACHN^EFORMIS, Woi'then. 

 PI. XXVIII. Fig. 12. 



Poteriocrinus arachnceformis, WOBTHEN, February, 1882. 



Bulletin No. 1, of the Illinois State Museum of Natural History, p. 13. 



Body small, obconical, length and breadth about equal, gradually 

 swelling from the base to the summit of the radial series. Basals 

 projecting more than half their length beyond the column, and 

 forming by themselves a low, pentagonal cup. SubracUals compar- 

 atively large, three hexagonal, and two heptagonal. Eadials smaller 

 than the subradials, the articulating scar semi-circular, and occu- 

 pying only about half the width of the plate, with a projecting 

 rirn around its outer margin. 



Only one anal plate is preserved in the specimen under descrip- 

 tion, which is about half as large as the largest of the subradials 

 between which it rests, and it extends up to the summit of the 

 right posterior radial. The left posterior subradial is truncated 

 at the summit for the support of the second anal, which is not 

 preserved. 



