INVERTEBRATES. 295 



Albany, p. 18), which is distinguished by having the interambulaeral plates all 

 distinctly imbricating from the dorsal side, and those of the ambulacral series 

 imbricating from below upwards. He places it as a subgenus under Archseo- 

 cidaris, but we should think it generically distinct, if indeed the differences are 

 not of more than generic importance. He says the ambulacral and the inter- 

 ambulaeral plates, with the exception of their singular imbricating character, 

 are arranged as in ArchsRoddaris ; but by an oversight he omits, both in 

 the subgeneric and specific descriptions, to mention the nature of the tuber- 

 cles of the interambulaeral plates. Judging from some imperfect speci- 

 mens we have seen, however, that show the remarkable imbricating character 

 of the plates alluded to, and which doubtless belong to LepidecMnus, there 

 were apparently no primary tubercles on any of the interambulaeral plates 

 of the under side of the fossil, excepting the marginal rows. If this is 

 so, this type is very near Perischodomus of McCoy, and may be found to belong 

 to the same group. It is true the imbricating character of the plates is not 

 mentioned in the description of the fossil described by Prof. McCoy, but 

 owing to the fact that only a single very imperfect specimen of it has, we 

 believe, yet been found, this character might have been overlooked. 



We have not seen specimens of ALrcheeocidaris showing very clearly the 

 number of rows of interambulaeral plates. In several specimens we have been 

 able to count four rows, but as these specimens did not show the middle por- 

 tions of the fossil, it is highly probably there were several additional rows. 

 Judging from analogy, there was probably always an odd number of these 

 plates. As in other types of the suborder, the inner interambulaeral plates 

 are hexagonal in form, and the marginal rows pentagonal. 



According to Desor, this genus is peculiar to the Carboniferous system. It 

 occurs in both the upper and lower divisions ; and in the eastern parts of Kan- 

 sas, it is found in beds referred by Prof. Swallow to the Permian. The Euro- 

 pean Permian species originally placed in this genus, are referred by M. Desor 

 to his genus Eocidaris. 



Arch^eocidaris mucronata, M. and "W. 



PI. 23, Fig. 3 a, 3 b, 3 c. 



Arehxocidaris mueronata, Meek and Worthen, Sept., 1860. Proceedings Acad. Nat. 

 Sci., Philad., p. 395. 



Primary spines long, gradually tapering, slightly compressed 

 or nearly round, and apparently a little curved a short distance 



