FOSSILS OF TTIE BTTRLTXGTOX GROUP. 4M 



AY*- have been somewhat puzzled in regard to the generic characters 

 of this species. Its interambulacral plates are each provided with the 

 large central tubercle and spine, characterizing Arcluvoeidarift and.7oci- 

 ddfix. though these tubercles agree with those of Eoc-idaria in having 

 no ring or slight projection around the base, as in Archceocidaru. At 

 first we were inclined to believe it related to Prof. HALL'S genus Lepi- 

 dechinus, on account of the decidedly imbricating character of its plates; 

 but judging from the brief published description of the type of that 

 group (which has not yet been figured,) it would seem to belong even to 

 a different family or sub-family, as nothing is said in the description of 

 that type in regard to a large central tubercle for the articulation of a 

 larger spine on each of the interambulacral plates, the surface being, on 

 the contrary, merely described as crowded with " irregular granules." In 

 addition to this Prof. HALL places his group as a subgenus under Palce- 

 chinus, one of the distinguishing features of which is the presence of 

 numerous small, i in perforate tubercles covering all the pktes. without 

 any larger central perforated tubercle. 



It is worthy of note, however, that Prof. HALL has since figured 

 another species (L. nirisptnus, Twentieth Rep. Regents' Univ. X. Y. on 

 State Cab. N. H.) in which some of the interambulacral plates are pro- 

 vided with a large central tubercle, while others apparently have none. 

 We are not informed whether these larger tubercles are perforated at 

 the end, as in Archveocidaris and Eocidaris, or whether they are without 

 such perforation, as in Palaechinus; but if it belongs to the same family 

 as that including Palcecliinns, of which Lepidechinuais supposed to be a 

 sub-genus, it is almost certain that its tubercles are not perforated. 



At any rate, our fossil differs from L. rarispinus in the following char- 

 acters, that seem to be of more than specific importance. In the first 

 place, it differs materially in the very irregular nature of its ambu- 

 lacral 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 clearly but a single row on 

 each side, with each piece extending entirely across from the mesial 

 suture to the lateral margins. Again, it differs in having a large cen- 

 tral tubercle and spine on each one of all the plates of the entire inter- 

 ambulacral series. The lateral imbrication of the plates in our type 

 is also different, the direction of the imbrication being inward, except- 

 ing 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 marked feature throughout ; the lapping- 

 edges being- somestimes at least one-fourth the entire breadth of these 



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