INVERTEBRATES. 333 



ambulacral plates upon either side. The exposed surface of each 

 plate is quadrangular, and usually square, or nearly so. The sur- 

 face is covered with small granules for the articulation of minute 

 needle-like spines, the articular ends of which are marked by a 

 slight swelling. The jaws are remarkably large, and seem to have 

 formed an ovate body of ten or twelve pieces, each of which is 

 deeply furrowed at the ends, and perforated in the central region. 



Other openings and characters of this species cannot be correctly 

 denned from our specimens. Those represented by figures 5 and 6 

 show only the crushed apical regions and the parts described, and 

 were collected from the Chester limestone on Prairie du Long creek, 

 in Monroe county, by A. H. Worthen. The one represented by fig. 

 7 was obtained from the same horizon in Pope county, and may or 

 may not belong to this species. The jaws seem to be smaller than 

 they are in the other two specimens, but this may result from their 

 being more fragmentary. One genital plate, however, appears upon 

 this specimen having four pores, and if it belongs to this species it 

 will of course add that additional character. 



No. 2,481. Illinois State collection. 



PERISHODOMUS, McCoy. 



PERISCHODOMUS ILLINOIENSIS, Sp. nov. 



PI. XXXI. flg. 8. view of the apical end of a distorted specimen, natural size. 



General form apparently subspheroidal, and as there are five am- 

 bulacral and five interambulacral areas, and the former are so nar- 

 row, it probably has a subpentagonal outline as noticed by McCoy 

 in diagnosing the genus. 



Ambulacral areas narrow, depressed, over-lapped on either side by 

 interambulacral plates, and consisting of a double series of inter- 

 locking transversely elongated plates, each one of which is pierced 

 by a single pair, of pores. These plates are not uniform in size nor 

 shape, though most of them seem to have imperfectly defined sub- 

 pentagonal outlines. They slightly overlap from above downward. 



Interambulacral areas covering nearly the entire surface, but as 

 the equatorial region is not preserved, it is impossible to state the 

 number of ranges of plates in each area. There may be five and 

 there may be seven only five can be distinctly determined from the 

 apical view of our specimen. The plates are very irregular in size, 



