LEPTODIADEMA PURPUREUM. 131 



an outer row of larger plates and an inner row of small plates surrounding 

 the anal opening. 



The two anterior genitals are roughly heptagonal, the right and left pos- 

 terior genitals are hexagonal, and the odd posterior genital is pentagonal. In all 

 five the genital openings are well developed and surrounded by a raised ring. 



The actinostome is somewhat larger than the abactinal system ; actinal 

 cuts slight; buccal membrane closely covered with narrow elongate plates, 

 as in young Diadema, arranged in five wedge-shaped, ambulacral divisions; 

 buccal plates large, in five approximated pairs. Primary radioles about half 

 the diameter of the test, those of the ambulacra scarcely shorter or more 

 slender than the others; all are delicate, glassy, slightly curved, and blunt, with 

 five to seven prominent ridges, two of which (on opposite sides of the spine) 

 may bear, at least near the base, a few widely separated, very slender teeth. 



Leptodiadema purpureum A. Ag. and CI. 



Leptodiadema purpureum A. Agassiz and Clark, 1907. Bull. M. C. Z., L, p. 239. 

 Plates 50, figs. 20, 21 ; 55, figs. 7-10. 



The single specimen obtained is 9 mm. in diameter with 13-14 cor- 

 onal, interambulacral plates. The color is dull purplish, becoming bright 

 purple on the buccal membrane. The spines are nearly colorless. The 

 two anterior genital plates are larger than the posterior ones. The genital 

 openings are nearly in the centre of the irregularly pentagonal plates, 

 and are surrounded by a small protuberance. The madreporic body is well 

 marked (PI. 55, fig. 8), The oculars are pentagonal or hexagonal, and the 

 three anterior are excluded from the anal system. The distal part of the 

 actinal system beyond the five pairs of large buccal plates is covered with 

 long narrow plates arranged in five separate wedge-shaped groups, one in 

 each ambulacrum. On the actinal side of the test the primary tubercles of 

 both the ambulacral and interambulacral areas are nearly of the same size 

 (PL 55, fig. 7) ; but above the ambitus the tubercles of the ambulacral sys- 

 tem are quite small (PL 55, fig. 8). The median abactinal part of the 

 interambulacral area is nearly bare, only a very few miliaries and second- 

 aries being carried by the four uppermost abactinal plates. 



No pedicellarise of any kind were present in the single specimen of this 

 species, possibly further evidence of its immaturity. 



