GENERA. AND GROUPS OF THE ECHINOIDEA. 11 



Genus Arch^ocidaris, McCoy, 1844, Si/nop. Carl. JFoss. Irel. 

 p. 173. TrautschoU, 186S, Bull. Moscou, vol. xli. p. 467. 

 J. Young, 1873, Geol. Mag. vol. x. p. 302; Proc. Nat. 

 Hist. Soc. Qlasg. vol. ii. pt. 2, p. 325. B. Etheridge, Junr., 

 1874, Quart. Journ. Geol. Soc. vol. xxx. p. 311. Loven, 1874, 

 Btucles, p. 43. W. Keeping, 1875, Quart. Journ. Genl. Soc. 

 vol. xxxii. p. 39. A. Agassiz, 1881, ' Challenger' Beport, 

 p. 77. (With additions.) 



Syn. Echinocrinus, Ag. ; Palcsocidaris, Deaor. 



Test large. Shape and apical system unknown. 



Ambulacra narrow, straight, reaching beyond the peristome to 

 the true moutb ; plates irregular, imbricating adoraliy, each 

 perforated by two pores ; plates in two vertical rows. 



Interradia with from three to five vertical rows of large, thin 

 plates, the median hexagonal and those of the rows nearest the 

 ambulacra more or less pentagonal, diminishing in number 

 towards the poles, and continued beyond the peristome as 

 small plates. The median plates are bevelled over those on 

 either side slightly, and these over others to the ambulacral edge, 

 which may be raised ; the adoral edge of each plate with a broad 

 groove for the reception of the corresponding aboral process of 

 the next plate situated adoraliy ; the plates within the peristome 

 overlap towards the apex. Slits at the peristome for the external 

 branchiae. 



Jaws with long, broad, grooved teeth, the pyramids rather 

 short, upper foramen small, cheeks deeply cut. 



A large primary tubercle upon each interradial plate, having a 

 low, conical truncated boss, supporting a narrower subcorneal per- 

 forated mamelon, surrounded by a wide groove ; scrobicule large, 

 almost flat ; plate beyond with a circlet of secondary tubercles and 

 large granules, with concentric striations or crenulations. Spines 

 of the large tubercles large, long, slender, bluntly serrated on the 

 longitudinal ribs, some spines smooth, and beyond the lower third 

 striated and with rows of oblique spinules. The annular ridge of 

 a spine may be crenulated. 



Fossil. Carboniferous Limestone : Ireland, Scotland, England, 

 Wales, Europe, and N. America (Upper Coal-measures, Illinois). 

 Permian : England. 



