110 HAWAIIAN AND OTHER PACIFIC ECHINI. 



Finally the hollow, verticillate spines and perforate tubercles show a close 

 relationship to the Diadematidas. In view of all these facts, we feel 

 obliged to reject the family Micropygidaj, since its recognition seems to 

 us to involve a most unnatural separation of Micropyga from its nearest 

 allies. 



In addition to Micropyga, we find it desirable to recognize eight genera 

 of recent Diadematidoe, of which Diadema, Echinothrix, Centrostephanus, and 

 Astropyga are old and well-established. There can be no question, either, as 

 to the validity of Chsetodiadema, but Lissodiadema and Leptodiadema are 

 both based on very small specimens and their real status is not perfectly clear. 

 As the eighth genus, we wish to establish Eremopyga, based on Astropi/ga 

 denudaia de Meijere, from the Dutch East Indies. This handsome " Siboga " 

 echinoid has hollow spines, few primary tubercles in the interambulacra, a 

 primary tubercle on each ambulacral plate (at least near the ambitus) and nar- 

 row poriferous zones with the arcs of pores in a nearly vertical series ; while 

 Astropyga has solid spines, many primary tubercles in the interambulacra, a 

 primary tubercle only on every second or third ambulacral plate (even at the 

 ambitus), and broad poriferous zones with the arcs of pores decidedly oblique. 

 These differences appear to us to be too important to warrant placing 

 deimdata in Astropyga. 



The genera we recognize are distinguished from each other largely by 

 the structure of the primary spines and the distribution of the primary 

 tubercles, but the form and structure of the test, the presence of spines on 

 the buccal plates, the width of the poriferous zones, the crenulation of the 

 tubercles, the size of the actinostorne, the structure of the pedicellarise and 

 even the calcareous particles in the pedicels, furnish more or less important 

 and usable characters. The following table will make the differences 

 clear. 



Primary spines of interambulacra rough with minute teeth, which are 

 arranged either in whorls or in crowded longitudinal series. 



Normal primary spines hollow for the greater part of their length. 

 Test moderately thick, more or less flattened, with vertical diam- 

 eter about half horizontal, always exceeding .40 h. d., some- 

 times over .60. 



Buccal jjlates with few or no spines ; no globiferous pedi- 

 cellariae. 



Ambulacra with few or no secondary tubercles abacti- 

 nally and narrower tliere than at ambitus ; ambulacral 

 primary spines not essentially different from those 

 of interambulacra Bladema. 



