Boone, Echinoidea, Cruise of "Alva," 1931 259 



Distribution : This species is widely distributed in the south- 

 ern Indo-Pacific from eastern South Africa to the Paumotu Archi- 

 pelago. It is recorded from the Zanzibar coast, Indian Ocean, 

 Mauritius, Philippine Islands, New Guinea, Caroline Islands, Gil- 

 bert Islands, Marshall Islands, Fiji Archipelago, Baker's Island, 

 Tongatabu, Society Islands, and Paumotus. It is restricted to the 

 littoral zone. 



Material examined: One very large specimen from Muller's 

 Reef, Bora-Bora, Society Islands, August 25, 1931. Four small to 

 medium-sized specimens from Venus Point Reef, Society Islands, 

 August 9, 1931. 



Colour: This species shows considerable diversity of colour, 

 according to the records. Some are reported to have brown test 

 and spines with shadings of green and orange-red on the pri- 

 maries. Others from the Society and Paumotu Islands are deep 

 purple, or of varying shades of purple. Some of the "Alva" speci- 

 mens, from the Society Islands, are deep purple, while others are 

 brownish with a greenish cast ; these latter are apparently more 

 faded by the alcohol than the royal purple specimens, some of 

 which retain the deep purple streaked with brownish where the 

 purple has very obviously faded. 



Discussion : This genus of well-known "slate-pencil" urchins 

 has but two valid species, H. mamillatus, the genotype, which is 

 more northern in its distribution, and the present species, H. trigo- 

 narius Brandt, which possesses an even greater individual di- 

 versity in several characters than does the genotype. The most 

 reliable diagnostic feature of H. trigonarius is to be found in the 

 fact that the pore-pairs in mid-zone, are in arcs of eleven (ten to 

 twelve), while those of H. mamillatus in mid-zone are in arcs of 

 fifteen (variation fourteen to nineteen) . The secondary spines of 

 trigonarius are short, normally pointed and tapering, while in 

 mamillatus these are usually stout and bluntly truncated. The 

 secondaries of trigonarius show more diversity, occasionally be- 

 ing as stout and truncate distally as those of mamillatus. As 

 shown in the plates 95 and 96, the primary spines of trigonarius 

 vary from long, thick, trigonal with tapering and subacute apices 

 to short, clavate, distally, truncate ones. H. trigonarius has nor- 

 mal primary tubercles in the abactinal region, while mamillatus 

 has none here. 



