28 PEOF. p. M. Duncan's EEvisioisr ov the 



cidaris, Desor * ; PJiyllacantJius, Brandt f ; Dorocidaris, A. 

 Agassiz %. 



Test variable in size and thickness, spheroidal, depressed or tall. 



Apical system large, either flat, and with the large geometrical 

 basal plates weakly united, or solidly sutured ; the radial plates 

 large, separating the basals or not, and in the first case often 

 touching the outer periproctal plates which may intrude. Basal 

 pores large ; the madreporite in the right anterior basal plate ; 

 the periproct pentagonal, variable in size, numerously plated. 



Ambulacra narrow, undulating or nearly straight, composed of 

 very numerous broad low primaries, which are perforated by 

 single pairs of pores, the pores being variable in their distance, 

 and may or may not be united by a groove ; poriferous zones 

 broad ; interporiferous areas with vertical rows of distinct granules 

 which are small, and may have rudimentary mamelons. The 

 plates are continued beyond the peristomial margin to the edge 

 of the mouth, and are low, broad, imbricating, and each is pierced 

 by a pair of pores. 



Interradia broad ; coronal plates few, from 5 to 11, in each of 

 the two vertical rows, each with a primary tubercle, scrobiculate, 

 and perforate, may be crenulated or not. Scrobicule large, often 

 sunken, its margin with small secondary tubercles and large 

 granules ; the space beyond the scrobicules with large miliary 

 granules. 



Peristome large, without branchial incisions ; the interradial 

 plates continued beyond the peristome to the mouth, small, low, 

 imbricating, in double series. Spheridia absent. 



Perignathic girdle discontinuous ; auricular processes upon 

 the interradia. Pyramids of jaws unclosed above by epiphyses ; 

 teeth grooved. 



Spines of the ambulacra, the scrobicular circle, the miliary 

 zones, and of the plates beyond the peristome very similar, small, 

 straight, blade~of-oar-shaped, very close ; and those of the scro- 

 bicules cling around the primary spines. Primary spines very 

 variable, even in the same species, with a calcareous network 

 internally and solid outside, lamellary, or fluted externally, and 

 variously spinuled, granulate, laminated, some whorled; long, with 

 a cylindrical or angular section, pointed or blunt ; or very short, 



* Synopsis, p. 48. 



t Prodr. d. Act. d. I'Aead. St. Petersb. 1834 (Additions), p. 267. 



:j: Revision, p. 254. 



