INVERTEBRATES. 305 



GENUS RHODOCRLNTJS, Miller. 

 RHODOCEINUS COXANUS, Worthen. 



PI. XXVIII, Fig. 7. 



Rhndocrinus Coxanus, WOKTHEN, Feb. 1882. 



Bulletin No. 1, of the Illinois State Museum of Natural History, p. 30. 



Body of medium size, subglobose, base slightly depressed, the basal 

 plates concealed by the first columnar joints. Subradials a little 

 wider than long, their lower angles curved in to form a part of the 

 basal concavity. 



The first radials much larger than the succeeding ones, heptagonal 

 in form; second radials nearly quadrangular, and the third hexag- 

 onal, supporting above the secondary radial plates, of which there 

 is but one to each division of the ray. 



Brachials, four to five, short, widening above, the upper one with 

 a slightly- projecting angle in the middle, separating the first divisions 

 of the arms. From two to five single, wedge-formed pieces succeed 

 the brachials, above which the arms are composed of a double series 

 of short, interlocking pieces, each one of which gives off a moder- 

 ately-strong pinnule. In one of the specimens under examination 

 the arms on the two rays visible continue single after the first bi- 

 furcation, making four arms to each ray; but in another specimen, 

 apparently of the same species, a third branch is given off from the 

 third plate above the first division, making six arms to these two 

 rays. 



From four to six inter-radials can be seen, the first of which is 

 larger than the others, but owing to the crushed condition of the 

 specimens, the exact form and number of these plates cannot be 

 determined. 



Column round, rather stout, and composed of alternately thin and 

 thicker joints. 



The specimen showing six arms to the ray also shows a marked 

 prominence in the central portion of the body-plates, which is not 

 seen in the other, and this we take as the type of R. Coxanus, and 

 if these differences should be regarded as of specific importance, 

 the name R . polydactylus might be used to designate the other va- 

 riety. 



Geological position and locality: Upper part of the geode bed, one 

 mile below Keokuk. 



Nos. 197 and 223 of Mr. L. A. Cox's collection. 



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