328 PALEONTOLOGY OF ILLINOIS. 



each side of the ambulacral furrows, which are rounded upon the 

 exterior, prolonged below, and each bear a short, sharp spine on the 

 upper part of the inner side, directed toward the apex of the ray. 

 There are about eighty adambulacrals upon each side of the furrows, 

 and they dimmish very slowly in size toward the apex, after passing 

 the fusiform enlargement of the ray. 



The disc-plates abut upon the adambulacrals in a straight line ; 

 they are subquadrangular, or polygonal, and each bears a short, 

 strong central spine. Six longitudinal rows appear on the ventral 

 side on each side of the ambulacral furrows ; they are somewhat 

 uniform in size, arranged in transverse, as well as longitudinal order, 

 and are about two-thirds as numerous in a longitudinal row as the 

 plates in an ambulacral series. 



Oral plates, madreporiform tubercle, and dorsal side unknown, ex- 

 cept so far as we may judge of the spinous character of the plates 

 from those visible on the ventral side. 



This large and beautiful species is so distinct from any hitherto 

 described, that no comparison is necessary for the purpose of iden- 

 tification. 



Position and locality: Okaw bluffs, between Chester and Kaskas- 

 kia, Eandolph county, Illinois; from the second division of the 

 Chester limestone. 



Illinois State collection of 1880, by A. H. Worthen, No. 2,476. 



CHOLASTER n. gen. 



(Ety. cholos, defective; aster, a star.) 



Body truncated pentagonal, central area circular, deep and large 

 in proportion to the rays ; rays distant, small, short and abruptly 

 truncated. 



Dorsal side possessing a rather large centro-dorsal plate, which is 

 surrounded by five plates occupying the position of radials, while 

 the other part is covered with numerous small disc plates. 



This genus being founded on a specimen showing the dorsal side 

 only, further particulars will be found in the definition of the species. 

 This fossil is so widely different from those heretofore discovered 

 that we think it belongs to a distinct family, which might, very 

 appropriately, be designated the Cholasteridte, but until other speci- 

 mens have been found showing other parts of the body, we prefer 

 to let the family remain undefined. 



