430 HETEROCENTROTUS TRIGONARIUS. 



extremity, or nearly black spines of uniform coloration, with all possible 

 combinations of these two extremes ; the most common coloration being 

 a light chestnut-color with straw-colored rings at the extremity of the 

 spines. 



The number of pores in each arc is small in this species, generally from 

 ten to eleven pairs ; while in II. trigonarius they range, in the same-sized 

 specimens, from fifteen to seventeen pairs. 



Red Sea; East India [stands; Feejee [slands; Sandwich Islands. 



Heterocentrotus trigonarius 



! Echinus trigonarius Lamk., 1816, An. s. Vert. 

 Heterocentrotus tritjnimrins Hit . is:;.",. IVud. \)<<c. 



PL III'./. 6 ; PL VI. f. ;.;. 



The tubercles of the interambulacral space are placed close together, 

 leaving but little space both in the median interambulacral space and along 

 the sutures for the secondary tubercles. These are proportionally small, few 

 in number, many of them reduced to miliaries. The secondaries cany spines 

 similar in shape to the primary spines, but shorter, so that the space between 

 the large primary spines is tilled by short, sharp-pointed, secondary spines 

 and thin, tlat. miliary spines; the secondary spines are only rarely trun- 

 cated, rather bevelled at the extremity, thus forming an interstitial filling 

 of an entirely different sort from that of II. mammillatus. The abactinal 

 system is somewhat less prominent, being more or less hidden by the closer 

 granulation covering it. The striking specific feature is the structure of the 

 ambulacra! system. The two vertical rows of primary tubercles extend to 

 the abactinal system, gradually diminishing in size from the ambitus towards 

 the ocular plates, and diminishing somewhat faster than the corresponding 

 tubercles of the interambulacral area. The most common coloration of the 

 spines of this species is a uniform dark violet, or with a few lighter bands at 

 the extremity of the spines. The flat, fan-shaped spines of the lower sur- 

 face are generally of a lighter color and frequently banded at the extrem- 

 ity on the lower side of the spines. The same is also the case with the 



