DIADEMA. Ill 



Ambulacra with numerous secondary tubercles abacti- 

 nally and distinctly wider there than at ambitus ; am- 

 bulacra! spines filiform, smooth except near tip . . Echinothrix. 

 Buccal plates with numerous spines; globiferous pedicel- 



lariee present Centrostephanus. 



Test thin, much flattened, with vertical diameter about one-third 

 horizontal and never exceeding .40 h.d. 



Pores distinctly biserial; anchor-shaped spicules in pedi- 

 cels Mkrojnjga. 



Pores uniserial or nearly so ; no anchor-shaped spicules in 



pedicels Eremopyga. 



Normal primary spines with central cavity so filled by a calcareous 

 network that under low magnification they appear solid in cross- 

 section. 



Actinal surface with normal primary tubercles ; poriferous zones 

 becoming wider at peristome; primary tubercles in ambu- 

 lacra, near ambitus, only on every second or third plate ; 



actinostome, .25-.35 h. d Astropyga. 



Actinal surface with tubercles tending to become small and 

 densely crowded near peristome; poriferous zones at peris- 

 tome reduced to a single series of widely separated pairs of 

 pores ; primary tubercles on each ambulacral plate, at least 

 near ambitus; actinostome very small, only .15-25 h.d. . . . Chcetodiadema. 

 Primary spines of interambulacra nearly or quite smooth, even under high 

 magnification. 



Primary tubercles non-crenulate ; each coronal plate at ambitus with 



3 primary and 8 or 9 secondary tubercles Lissodiadema. 



Primary tubercles finely crenulate; each coronal plate at ambitus 



with not more than 2 primary and 4 or 5 secondary tubercles . . Leptodiadema. 



DiADEMA. 



Gray, 1825. Ann. Phil., p. 4. 

 Type-species, Eehinometra setosa, Leske, 1778. Add. Klein, p. 36. 



Few genera of Echini afford as much difficulty as does this one in the 

 way of distinguishing its component species, for while typical examples from 

 widel}^ separated localities look quite different and seem to be readily distin- 

 guishable, as soon as one attempts to state the differences it is found that 

 they are remarkably intangible. For example, the olive cast of color of 

 typical Hawaiian specimens is very different from the reddish tinge of cer- 

 tain examples from Zanzibar, and neither is at all like the deep purplish- 

 black of West Indian specimens. But when large series of specimens are 

 examined these differences of color are found to be very unreliable, and it 

 seems clear that no reliance can be placed on color for separating the species. 



