NATURAL SCIENCES OF PHILADELPHIA. 81 



Cab. N. H.) in which some of the interambulacral plates are provided with a 

 large central tubercle, while others apparently have none. We are not in- 

 formed whether these larger tubercles are perforated at the end, as in Archa- 

 ccidaris and Eosidaris, or whether they are without such perforation, as in 

 Palxchinus ; but if it belongs to the same family as that including Palxchimis, 

 of which Lepidechinus is supposed to be a sub-genus, it is almost certain that 

 its tubercles are not perforated. 



At any rate, our fossil differs from L. rarispinis in the following characters, 

 that seem to be of more than specific importance. In the first place, it differs 

 materially in the very irregular nature of its ambulacral pieces, which show a 

 strong tendency to pass into, and in fact do, at some places, actually pass into 

 two rows on each side of the mesial zigzag suture, instead of having clearlj' 

 but a single row on each side, with each piece extending entirely across from 

 the mesial suture to the lateral margins. Again, it dilTers in having a large 

 central tubercle and spine on each one of all the plate of the entire interambu- 

 lacral series. The lateral imbrication of the plates, in our type, is also different, 

 the direction of the imbrication being inward, excepting in the two outer 

 rows on each side, instead of outward in the whole series, the middle row 

 being clearly lapped on each side, instead of lapping those on each side of it. 



The strongly imbricating character, especially of the interambulacral plates 

 in our type, is a very raarlied feature throughout ; the lapping edges being 

 sometimes at least one-fourth the entire breadth of these 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 dis- 

 placed. We are not aware whether this imbricating character of the plates 

 has been observed in any of the European types on which the genera Eocida- 

 ris and Archxocidaris were founded.* It is certainly more or less marked, 

 however, in several of the American forms now before us that have been' re- 

 ferred 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. Shitmardi. 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 sup- 

 porting the primary spines, seen in the true Archieocidaris, and distinguishing 

 it from Eocidaris. It seems therefore probable that this character may be 

 more or less marked in both Archceocidaris and Eocidaris^ but most apparent in 

 species which, like that we have here described, are provided with plates of 

 more than the usual thickness. f 



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

 ture 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 (Twentiet)i 

 Report Regent's 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 description correctly. 



From all that is therefore known in regard to the several types mentioned, 

 we are much inclined to believe that our fossil will be found to belong either 

 to a distinct subgenus under Eocidaris, or to an allied new genus. In either 

 case we would propose for the group the name of Lepidocidans. 



Locality and position, — Lower beds of the Burlington group, Burlington, 

 Iowa. Lower Carboniferous. No. 404 of Mr. Wachsmuth'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 Archaocidaridie as well as in some of these older 

 types, apparently not belonging to that family. 



1869.J 6 



