112 HAWAIIAN AND OTHEE PACIFIC ECHINI. 



The same is true to a greater or less degree of all the other characters by 

 which it is customary to distinguish species, and we are finally driven to lay 

 weight on characters which seem trivial and of doubtful value. There seems 

 to be no doubt that the type-species (setosum) can always be distinguished 

 when mature ; it ranges from Zanzibar to Tahiti and perhaps even to Japan. 

 But other Diademas, which are certainly not setosum, occur throughout the 

 Indo-Pacific region, as well as in tlie West Indies, and on both coasts of 

 tropical America. Do these all represent a single species, or are there char- 

 acteristic forms in each of these widely separated areas ? After careful 

 study of large series of specimens, it has seemed better to us to try and dis- 

 tinguish five species than to mass all this material together under one name. 

 Accordingly we distinguish antillarutn Phil, from the tropical Atlantic, mexi- 

 canum A. Ag. from the West Coast of tropical America and the Galapagos 

 Islands, pmicispinum A. Ag. from the Hawaiian Islands, rjlobulosmn A. Ag. 

 (including nudum A. Ag.)-from the Gilbert and Society Islands and Hong 

 Kong, and Savignyi Mich, from the whole Indo-Pacific region, Zanzibar to 

 Easter Island. All of the species are littoral. The characters by which 

 they are distinguished are the arrangement of the primary tubercles in the 

 interambulacra abactinally, the character of the spines, the size of the abac- 

 tinal system, the depth of the actinal cuts, and the form of the tridentate 

 pedicellarite. The following table will show how these features are com- 

 bined in the six species which we recognize. 



Second series of interambulacral primary tubercles begins abactinally on 7th. 

 or Sth coronal plate ; spines slender, fragile, with 24-32 longitudinal series 

 of teeth; tridentate pedicellariee slender, with, narrow, compressed valves setosum. 

 Second series of tubercles begins abactinally on 4th, Sth, or 6th (rarely 7th) 

 coronal plate ; spines stouter, with 20-28 longitudinal series of teeth ; tri- 

 dentate pedicellariiB stout, with wide valves which are little or not at all 

 compressed. 



Abactinal system less than half diameter of actinostome. 



Actinal cuts usually deep and narrow ; secondary and miliary 

 tubercles rather few actinally ; valves of tridentate pedicellarioe 

 rather flat, nearly straight, with apophysis ending in a T  antillarum. 



Actinal cuts usually wide and shallow; secondary and miliary 

 tubercles rather numerous actinally ; valves of tridentate pedi- 

 cellarise slightly compressed at base of blade, wide near tip, 



curved, with apophysis ending in a Y mexicanum. 



Abactinal system more than half diameter of actinostome. 



Abactinal system .50-.55 of diameter of actinostome ; second series 

 of tubercles begins on 4th or 5th coronal plate ; ambulacra nar- 

 row, about ^ of interambulacra; valves of tridentate pedicellariaj 

 somewhat compressed though broad, not narrowed near tip . paucispinum. 



