378 HAWAIIAN AND OTHER PACIFIC ECHINI. 



confirms the opinion. But this examination also shows that the primary spines 

 are of no value whatever as a means of distinguishing the species, few Echini 

 showing so great variability in this respect. The secondary spines are of more 

 use, though they cannot be absolutely relied on. The best specific character is 

 found in the arcs of pores, the difference in this respect being remarkably con- 

 stant. A rather marked difference in the tridentate pedicellarise has been noticed 

 by Mortensen, but I have made no attempt to determine how constant this is. 

 By the use of these various characters, the two species maybe distinguished 

 as follows: 



Pore-pairs, in mid-zone, in arcs of 11 (10-12) ; secondary spines, short, flaring, and trun- 

 cated at tip; no large primary tubercles in abactinal part of ambulacra; valves 

 of tridentate pedicellarise somewhat curved, widened at tip where alone they are 

 in contact mammillatus. 



Pore-pairs, in mid-zone, in arcs of 15 (14-19); secondary spines, short, usually tapering 

 and pointed at tip; normal primary tubercles in abactinal part of ambulacra; 

 valves of tridentate pedicellariae bluntly pointed, in contact for most of their 

 length trigonarius. 



Heterocentrotus mammillatus Br. 



Echinus mammillatus Linne, 1758. Syst. Nat., ed. 10, p. 667. 



Heterocentrotus mammillatus Brandt, 1835. Prodrom. desc. Anim., p. 266 (or 66) 



Plates 115-117. 



The specimens of Heterocentrotus in the "Albatross" collection are divisible, 

 at a glance, into two groups on account of their color. In one group, the general 

 coloration is brown, the secondaries lighter than the primaries and usually light 

 fawn- or even cream-color; these specimens are all mammillatus and are all from 

 the Hawaiian Islands. In the other group the general coloration is deep purple 

 and there is little difference in shade between primaries and secondaries; these 

 specimens are all trigonarius and are from the Paumotus. So far as known 

 trigonarius does not occur in the Hawaiian Islands, nor mammillatus in the 

 Paumotus. Indeed the latter seems to be confined to the North Pacific and 

 Indian Oceans; the M. C. Z. has good series from Mauritius, the Bonin Islands, 

 and the Hawaiian Islands, but only one specimen from south of the equator 

 in the Pacific, and that one is a young individual from Jarvis Island (which is 

 less than 23' S.), the identification of which admits of some doubt. So far as 

 our material is concerned, mammillatus is often very light colored, and never 

 shows any indication of purple; when dark colored, the shade is always a rich 

 chocolate-brown. 



