520 PAL2EONTOLOGY OF ILLINOIS. 



posed of rather stout, round pieces, two or three times as long as wide. 

 The number -and arrangement of the arms of this species, which sug- 

 gested the specific name, will distinguish it from any other form at 

 present known from this horizon. 



Position and locality : Monroe county, Illinois, in the upper division 

 of the St. Louis group. Lower Carboniferous. 



Genus (XNTCHOCKINUS, L. and 0. 



Onychocbintjs magntjs, Worthen. 



PI. 31, Fig. 5. 



Body large, and composed of massive and apparently smooth plates. 

 Basal pieces small, and entirely concealed in the columnar depression. 

 Subradials of unequal size, the two on the anal side much smaller than 

 the others, and the three on the anterior side showing a pentagonal form 

 beyond the columnar facet. First radial pieces twice as wide as long, 

 hexagonal and keptagonal, counting the slight angular depression on 

 their upper margins; second, third aud fourth radials nearly as wide 

 and rather shorter than the first; fifth radials as long as the first, meas- 

 uring to the top of their upper angles, and supporting on their upper 

 sloping margins the first of a double series of secondary radials or 

 brachial pieces, of which about 27 or 28 can be counted in each series 

 on two of the rays, but gradually diminishing in size to their extremi- 

 ties, where they are no larger than their pinnulte. Each of the ten 

 arms gives off on alternate sides from the third, fourth or fifth plate 

 clusters of stout pinnulse composed of plates similar to those of the 

 arms, but gradually decreasing in size to their extremities. All the arm 

 plates, as well as those of the pinuulse, are angular below, the angle 

 fitting into a depression in the subordinate plate. A single anal plate 

 only is visible in our specimen, aud by an oversight this is not repre- 

 sented in the drawing. It is small, about twice as long as wide, and 

 apparently pentagonal. Column round, comparatively large at its 

 junction with the base, composed of very thin equal segments connect- 

 ing by crenulated sutures, and decreasing in diameter nearly one-half 

 in the space of an inch below the base. 



Position and locality: Monroe county, Illinois, in the upper division 

 of the St. Louis group. Lower Carboniferous. 



