PERISCHOECHINIDAE. 



Isolated plates of Sea-urchins, referred formerly to the genus Cidaris, have 

 been known for a long time from the Silurian, Devonian, and Carbonifer- 

 ous. Owing to their peculiar shape and the character of their tubereulation, 

 they were separated from Cidaris as Echinocrinus by Agassiz, and subse- 

 quently placed by him in the vicinity of Cidaris. McCoy, in his Carboniferous 

 Fossils of Ireland, was the first to describe tolerably complete specimens and 

 to throw new light on their affinities. 



A number of genera of this group have been discovered in America and 

 Europe ; the affinities of the group of Echinoderms comprising Melonites, 

 Archaeocidaris, Eocidaris, Palaechinus, etc., have been the subject of much 

 discussion, from the time of their first discovery, when they were considered 

 as true Echini, till McCoy proposed to consider them as a distinct older, 

 equivalent to the Echini, under the name of Perischoechinidae* Rocmer con- 

 sidered them a suborder t of Echini, and DcsorJ a tribe, to which he gave the 

 name of Tesselles, while formerly Professor Agassi/., on theoretical grounds 

 alone, and before the structure of this group had been as well marked out as 

 it is at present, considered them as belonging to Crinoids. Since the publica- 

 tion of these views a considerable number of more or less perfect specimens 

 have been found, until it is now possible to reconstruct the charac- 

 teristic features of this type of Echinoderms ; the comparison with the 

 recent genera of Cidaris has shown points of identity which will, I think, 

 place beyond doubt the true nature of these apparently anomalous Echino- 

 derms, and show plainly that their systematic position is that of a suborder 

 among Echini, as first correctly appreciated by Roemer. Before the dis- 

 covery of the oral and abactinal openings the presence of a large number 

 of plates in the interambulacral space seemed to relate them to the Cystidae, 

 where we find hexagonal, pentagonal, and other irregularly shaped plates, 

 many of which are perforated ; but, as Midler has clearly shown, their pores 

 cannot be homologized with those of the true Echini, being openings corre- 



* Brit. Palaeozoic Fossils, p. 124. 



t Wieg. Archiv.. 1855, p. 312. 



i Synopsis d. Eekinides Foss., 1858, p. 152. 



