TRIGONOCIDARIS ALBIDA. 295 



Actinal interambulacral plates not Ic ver than abactinal 

 nor noticeably different in shape; their primary 

 tubercles not arranged in horizontal series of 3 

 or 4; bare interambulacral space extended more 

 or less conspicuously along the horizontal sutures; 

 oculo-genital ring not conspicuously dark .... Microcyphus. 



Actinal interambulacral plates distinctly lower and 

 more oblong than abactinal; primary tubercles on 

 each one in a horizontal series of 3 or 4; bare inter- 

 ambulacral spaces not extended noticeably along 

 horizontal sutures; oculo-genital ring more or less 



black or blackish Salmacopsis. 



Median abactinal interambulacral areas not bare but more or 



less covered by secondary or miliary spines A mblypneustes. 



Only every second or third ambulacral plate at and above ambitus with 



a primary tubercle close beside the poriferous area. 



Ocular plates typically all exsert; interambulacral plates low and 

 numerous (27-30 in specimens 23 mm. h. d.), each with several 

 primary and some secondary tubercles; actinostomal membrane 

 naked save for buccal plates Holopneustes. 



Oculars I and V insert; interambulacral plates high and few (18 in 

 specimen 23 mm. h. d.) each with 1 small primary, and 2-5 well 

 scattered, small secondary tubercles; actinostomal membrane 

 with numerous small plates, especially inside the buccal circle . Goniopneustes. 



TRIGONOCIDARIS. 



A. Agassiz, 1869. Bull. M. C. Z., I, p. 263. 

 Type-species, Trigonocidaris albida A. Agassiz, 1869, 1. c. 



Although a second species of this genus, from the Hawaiian Islands, was 

 described in 1907 (A. Agassiz and Clark, Bull. M. C. Z., L, p. 242), comparison 

 of those specimens with a large series from the West Indies ^hows that the 

 peculiarities supposed to distinguish the Hawaiian form are not reliable but 

 are quite inconstant and the genus is thus monotypic. 



Trigonocidaris albida A. Ag. 



Trigonocidaris albida A. Agassiz, 1869. Bull. M. C. Z., I, p. 263. 

 Trigonocidaris albidoides A. Ag. and CI., 1907. Bull. M. C. Z., L, p. 242. 



Examination of a large series of specimens from the West Indies shows so 

 great diversity in the sculpturing of the test, in the number of spines on the 

 abactinal system and in the amount and distribution of the red coloring, that 

 it is impossible to satisfactorily distinguish the specimens collected by the 

 ''Albatross" in the Hawaiian Islands. This would seem to indicate that de Mei- 

 jere was quite correct in identifying as albida, the Trigonocidaris collected by 



