274 DIADEMA SJiTOSUM. 



DIADEMA. 



Diadema Sctiyn. 1 711. Thes. Imag. (Peters, emend.) 



Outline of test slightly pentagonal, flattened at both poles, test moderately 

 thin. The tubercles of the ambulacra somewhat smaller than those of the 

 interambulacra, arranged in two vertical rows. The tubercles of both areas are 

 crenulate, perforate. The poriferous zones are narrow, the median ambulacra! 

 region much broader than the poriferous zone, but the ambulacra narrow 

 compared to width of interambulacra, and often rising considerably above 

 them. The pores are arranged in simple pairs, forming arcs round the 

 adjacent tubercles. The spines are all of same nature, hollow, closely verti- 

 cillate, and very long upon both areas, often three and even four times the 

 diameter of test in length. The milled ring at base very large and prominent, 

 and rapidly tapering to the width of shaft, which is maintained to extremity. 

 The actinal system is large; there are ten broad cuts, not very deep, but 

 provided with thin processes diverging from them. The actinal membrane is 

 thin, barely strengthened by small limestone plates. The ambulacral suckers 

 are provided with suckers on lower surface, but above the ambitus, up to 

 the abactinal system, they are pointed as in Arbacia. 



The bare part of the sunken interambulacra extending from the genital 

 plates branches and extends on either side of the outer main tubercles. 

 The abactinal system is made up of five large, pointed, triangular genital 

 plates, the genital openings placed in a pit near outer extremity. The ocular 

 plates are small, regularly intercalated between the genital plates. The anal 

 system is covered by a thin naked membrane, only strengthened near genital 

 plates by a row of small plates, anal opening at end of a tube extending like 

 a proboscis three to four times the length of the anal system. Teeth large. 



Diadema setosum 



! Diadema setosa (Gray), 1825. An. Phil. p. 4 (non Rdmph). 



PL IP./. €-10; PL II\f. 6 ; PL TV.f. 1 ; PL VP.f. 5. 



I have been unable, after repeated examinations of a large series of speci- 

 mens from many localities, to distinguish more than two species of Diadema. 

 Bolsche had already called attention to his inability to separate Diadema 

 antillarum from Diadema Savignyi ; and I have to acknowledge that the 

 different species I attempted to discriminate from various localities in the 



