218 HAWAIIAN AND OTHER PACIFIC ECHINI. 



and only 1.8 mm. in thickness. In a specimen 30 mm. in diameter, there are 

 10 interambulacral and 15 ambulacral plates to a column. The abactinal 

 system is 15 mm. in diameter and the actinostome 11.5 mm. In the smallest 

 specimen the test is 15 mm. in diameter and nearly 8 high; the abactinal system 

 measures almost 8 mm. across and the actinostome 6 mm.; there are 8 inter- 

 ambulacral and 10 or 11 ambulacral plates to a column. 



The periproct is large, its diameter about half that of the abactinal system; 

 it is covered by upwards of five hundred small, rounded plates of which ten or a 

 dozen near the approximately central anus, are noticeably larger than the others 

 and carry a few miliary spines and pedicellari&3; there is no indication whatever 

 of an anal tube. The genital plates are large, broadly in contact with each other 

 and completely exclude all the oculars from the periproct. The genital pores 

 show such diversity as to indicate a sexual difference; in some specimens they 

 are moderately large, circular and near the centre of the plate while in the others 

 they are very small and occupy a notch in the distal angle of the plate, from 

 which a narrow, shallow but quite distinct groove runs down nearly or quite 

 to the ambitus. The madreporic genital is no larger than the others and the 

 pores occupy only a small area at the centre of the plate. Each plate carries 

 4 or 5 small spine-bearing tubercles near the periproct and 2 or 3 others may be 

 scattered on the plate; there are also a number of minute pedicellaria-bearing 

 granules which are hardly visible to the unaided eye. The ocular plates are 

 about one third as large as the genitals; the pore is near the distal margin and 

 proximal to it are 2 or 3 small tubercles. 



The interambulacral plates are high but the surface of each is largely covered 

 by the primary tubercle; at both the outer and inner ends however, there are 

 secondary and miliary tubercles, which are largest and most numerous on the 

 actinal plates. The ambulacral plates are nearly as high as they are wide, with 

 the rather large pore-pairs occupying most of the outer end, while the inner half 

 is covered by the primary tubercle; there are, however, some small secondaries 

 beside the median suture especially actinally, and miliaries occur wherever there 

 is room especially abactinally. The pore-pairs form a nearly vertical series, 

 though below the ambitus the "arcs of three" are evident. The ambulacra are 

 half as wide as the interambulacra at the ambitus. 



The buccal membrane carries a considerable number of small plates, but these 

 are chiefly in the ambulacra distal to the primordial ambulacrals. The latter 

 carry small spines as well as pedicellaris while the little plates carry pedicel- 

 lariae alone. The gills are relatively small and the cuts are broad and shallow. 



