408 DIADEMA SETOSUM. 



DIADEMA. 



Diadema S.iiyxv.. 1711, Then. Imag. (Pet. emend.) 

 (S lv Part II. p. 274.) 



Diadema mexicanum 



'.Diadema mexicanum A. Ac;., 18Ci, Bull. M. ('. Z., I. 



PI. XXXVIII. f. 3-6. 



As 1 bave already mentioned in Part II. of this Revision, I am unable to 

 discriminate more than two species of Diadema. Even the differences of 

 these two species are not great, amounting only to a difference in the pro- 

 portions of the actinostome and the test, of which comparative measurements 

 have been given on page '^75. The variations in the thickness and length of 

 the spines is so great that it is impossible to draw any permanent characters 

 from them : yet. as a whole, the spines of D. mexicanum are stouter, 

 though longer, as compared with diameter of test, than those of I), setosum, 

 which vary immensely, according to size and locality. The spines are usually 

 banded in young specimens of both species. Abactinal system is larger in 

 proportion than in I), setosum, with a larger anal system and larger genital 

 plates; poriferous /.one somewhat broader, and the outer low of interam- 

 bulacral tubercles, adjoining poriferous /.one. of uniform size, diminishing hut 

 little towards abactinal pole, and nearly reaching the ovarian plates without 

 material decrease in si/.e. Actinal cuts not deep, hut broad ; while usually 

 they are much narrower in specimens of I), setosum of the same si/.e. 

 The verticillations of spines are liner in I), setosum than in D. mexicanum. 

 Although a large number of specimens of all sizes of these two species 

 have been examined and compared. I am unable to give any more satis- 

 factory discrimination between them ; the pedicellariae do not help us in 

 the comparison. 



Acapulco ; Cape St. Lucas. 



Diadema setosum 



I Diadema setosa (Gray), 1825, Ann. Phil, (non RrjMPH.). 

 (Sec- Pari II. p. 274.) 



PI. IP./. 6-10; PI ir.f. 6 ; PL IV.f.i; PI VI. f. 15; PL VI". f. 5; 

 PL XXIV. f. 88-89; PL XXVII. f. :,; PI. XXVIII f. 5. 



West India Islands; Cape Verde Islands : Indian Ocean; Japan; Sandwich Islands; Feejec Island?. 



