GENERA AND GROUPS OF THE ECHINOIDEA. 61 



decent. Caribbean Sea, Cape Yerd, Indian Ocean, Eed Sea, 

 Japan, Pacific Ocean, W. coast of Central America. 



Subgenus Centrostephanus, Peters, 1853 (genus), Abli. d. k. 

 Ahad. d. Wiss. Berlin, 1854, pub. 1855, p. 109 (note). A. 

 Agass. 1874, Revision, p. 409. Duncan, 1885, Journ. Linn. 

 8oc. (ZooL), vol. xix. p. 110. 



Syn. Echinodiadema, Verr. ; Trichodiadema, A. Agass. 



Test thick, large or moderate, globular or depressed, circular 

 at the tumid ambitus. 



Apical system with basal plates separated by small, rectangular 

 periproctal plates, and by radial plates also. Periproct with 

 minute plates. Anal opening small. 



Ambulacra with two rows of tubercles and the plates as in 

 Diadema, but the poriferous zones may be sunken. 



Interradia twice as broad as the ambulacra ; four vertical rows 

 of perforate and crenulate tubercles, two rows being more pro- 

 minent than the others, all subequal ; median space ill-defined. 



Actinal membrane of the incised decagonal peristome with ten 

 large plates carrying spines, pedicellarise, and tentacles. Spines 

 hollow, thin, very verticillate and striated longitudinally, long and 

 slender, some small and club-shaped. 



Recent. Mediterranean, Canaries, Australia, New Caledonia, 

 W. coast of Central America. 



Former genus Pseudodiadema, Desor, 1858, Synopsis, p. 63, pars. 

 Duncan, 1885, Quart. Journ. Geol. Soc. vol. xli. p. 430. Cotteau, 

 1864, Rev. etMag. de Zool. 2 ser. vol. xvi. p. 288. (Now Diadema.) 



Syn, Tetragramma, Agass. ; Hebertia, Mich. 



Test small and moderate in size, thin, tumid at the circular ambitusj 

 swollen to subcorneal dorsally, depressed, flat or re-entering actinally. 



Apical system small, ornamented ; the periproct circular or somewhat 

 elongate, and even unsymmetrical ; basal plates narrow ; radial plates 

 small, equal or unequal, and some may enter the periproctal ring ; the 

 pore close to the adoral edge ; madreporite distinct and in the right anterior 

 basal plate. 



Ambulacra much narrower than the interradia; plates as single low 

 primaries near the apex and soon becoming compound, the central com- 

 ponent with the tubercle, a primary, and the adoral and aboral constituents 

 also primaries, their sutures curved, convexity towards the tubercle ; some- 

 times a demi-plate near the peristome. Pairs of pores in simple straight 

 series near the apex and in the same towards the ambitus, or in slight arcs 



