334 PALEONTOLOGY OF ILLINOIS. 



imbricate upward, and the two rows adjoining the ambulacral areas 

 imbricate outward, but whether the imbrication is from the central 

 range outward, as is usual in such cases, cannot be determined, for 

 the central range seems to be depressed, which may have resulted 

 from the crushing the fossil has received. 



The plates bore two kinds of spines ; those bearing the larger kind 

 possessed a single central spine, but those bearing the smaller kind 

 possessed two or more subrnarginal ones. The central supporting 

 tubercle is a moderately large smooth hemispherical nipple with a 

 central perforation. Only one plate of this kind is shown upon the 

 typical specimen, and it is in the second range from the ambulacral 

 area at about the sixth plate from the genitals. The secondary 

 spines were borne by both rows of plates on each side of the am- 

 bulacrals, as indicated by small, more or less conical elevations, 

 some of which show a minute perforation, and indeed may have been 

 borne by the plates in the other ranges. Other characters of the 

 species unknown. 



The plates, in the interambulacral areas of this species, correspon d 

 in number, order of arrangement, and spinous character so much 

 more nearly with Perischodomus than with Lepidechinus, the most 

 nearly approximating genus in other known respects, that we have 

 little doubt of the correctness of the generic reference, though it 

 would be unnecessary to point out how it differs from the European 

 forms for the purpose of specific distinction. 



Collected by A. H. Worthen from the Chester limestone at Bay 

 City, Pope county, Illinois. 



No. 2,483 Illinois State collection. 



