482 PALAEONTOLOGY OF ILLINOIS. 



pieces, and yet, owing to the accuracy with which they are beveled, 

 they lie so evenly together that this peculiarity is scarcely apparent 

 where the plates have not been displaced. We are not aware whether 

 this imbricating character of the plates has been observed in any of 

 the European types on which the genera Eocidaris and Archceocidaris 

 were founded.* It is certainly more or less marked, however, in sev- 

 eral of the American forms now before us that have been referred to 

 the latter genus. For instance, it is clearly seen in the typical speci- 

 men of A. Agassizi, of Hall, and less distinctly in his A. S'humardi. 

 We can also see indications of it in A. Wortheni, of Hall, though owing 

 to the thinness of its plates the beveling of their edges is less apparent. 

 All these species have the peculiar ring or prominence around the base 

 of the tubercles supporting the primary spines, seen in the true Archoe- 

 ocidaris, and distinguishing it from Eocidaris. It seems, therefore, 

 probable that this character may be more or less marked in both Archce- 

 ocidaris and Eocidaris, but most apparent in species which, like that we 

 have here described, are provided with plates of more than the usual 

 thickness.t 



So far as we are aware, no European species of Eocidaris showing the 

 structure and arrangement of the ambulacral series of pieces has been 

 discovered ; at least we have seen no figures or descriptions of such. 

 Prof. HALL, however, has described a species from the Chemung group 

 of New York (Twent. Eep. Eegents' Univ., p. 298,) to which VANUXEM 

 had given the name Echinus Drydenensis, but which is said to be an 

 Eocidaris. In this the ambulacral plates are described as being in two 

 ranges without any intercalated pieces, if we understand the descrip- 

 tion correctly. 



From all that is therefore known in regard to the several types men- 

 tioned, we are much inclined to believe that our fossil will be found to 

 belong either to a distinct sub-genus under Eocidaris or to an allied 

 new genus. In either case we would propose for the group the name of 

 Lepidocidaris. 



Locality and position Lower bed of the Burlington group, of the 

 Lower Carboniferous, at Burlington, Iowa. No. 404 of Mr. WACHS- 

 MUTH'S collection. 



* Some of Prof. DESOR'S figures of the plates of Eocidaris appear to show indications of marginal 

 beveling, while others do not. 



t Since this was written we have been led to believe this imbricating character of the plates is more 

 or less defined in all the Archceocidaridce as well as in some of these older types apparently not belong- 

 ing to that family. 



