DIADEMATIDAE. 407 



by radiating ridges, round outer edge of genital plates and inner point 

 of ocular plates; in the only specimen existing, brought up on a fishing- 

 line from a depth of 200 metres, the anal plates as well as the buccal mem- 

 brane were wanting. The spines of the primary tubercles are immense, 

 totally unlike the spines of any other Echini, three times as long as diameter 

 of test; they are curved, triangular at base, with one rounded side, tapering 

 very gradually ; the milled ring is oblique; articulating boss smaller than 

 shaft of spine ; they are pinkish at base ; body of shaft greenish, with purple 

 bands. The spines surrounding actinostome are short, straight, flattened, 

 only banded on one side, resembling spines of Arbacia. The miliary spines 

 are narrow, cylindrical, longitudinally striated by minute serrations, usually 

 of a brilliant purple ; jaws resembling those of Arbacia. 



Bourbon. 



DIADEMATIDAE. 



Family Diadematidae Peters, 1853, Monatsb. Akad. Bed. (emend.) 



The test is thin, the ambulacra narrow ; ambulacral suckers pointed near 

 upper abactinal pole ; interambulacra depressed near abactinal system. 

 Spines long, hollow, verticillate ; tubercles of both areas similar in struc- 

 ture ; actinal cuts moderately developed, and in one genus ambulacra ex- 

 tending to the mouth, as in the Ciclaridae. The jaws and apophyses send 

 out thin processes, not forming connected arcs; teeth channelled as in 

 Cidaridae. 



The arrangement of the pores of Diadematidae when young is identical 

 with the arrangement of the pores found in such genera as Pseudodiadema, 

 Cyphosoma, Hemipedina, in which we are unable to say whether they belong 

 to the Oligoporidae or Polyporidae, forming, as they do sometimes, arcs of 

 three or four pairs of pores round the base of the primary tubercles ; showing 

 in young specimens, measuring 12 mm already, but a faint trace near actino- 

 stome of the arrangement of the pores in arcs of three. The abactinal 

 system is also remarkably different in structure from that of the adult, the 

 genital plates resembling more the pentagonal plates of the genital system 

 of Centrostephanus, and becoming pointed only with advancing age ; the 

 base of the anal membrane adjoining the ring of genital plates is strength- 

 ened by a row of small plates, which disappear in the older specimens or 

 are resorbed in the anal membrane. 



