24 TROPICAL PACIFIC ECHINI. 



tubercles which extend to the actinostome (Pis. 8, fig. 1; 9, fig. 1; 12, 

 fig.i). 



In the abactinal system of P. pedifera the genital plates (PI. 14, 

 fig. 3) carry from two to three large secondary tubercles with one to three 

 very small secondaries or miliaries in the angles of the plates adjoining the 

 anal system . The genital plates are irregularly heptagonal, the anterior plates 

 elongate, the posterior broader than long. The ocular plates are pentagonal 

 with a broad base curved towards the ambulacral system. They carry one 

 large secondary tubercle, sometimes also a couple of miliaries, and have a 

 well-marked ocular pore. 



The madreporic genital is elongate, carries three large secondary tubercles, 

 one in the centre of the plate, the others near the anal system, with five to 

 six small secondaries. The madreporite occupies the greater part of the geni- 

 tal. Seen from above, covered with radioles, the abactinal system (PI. 14, 

 fig. J) forms a close pavement of irregular pentagonal plates which completely 

 hide the anal system. 



The genital pores are large. They occupy the distal points of the genital 

 plates. They are well seen on PI. 14, fig. Jf, an interior view of the abactinal 

 system. This figure also shows the simple arrangement of the abactinal 

 pairs of pores above the few intercalated demi-plates. The anal system is cir- 

 cular, covered with three to four rows of small, irregular, elliptical plates. 



There is a great contrast between the genital plates of P. pedifera (PL 14, 

 fig. 3) with its three large secondary tubercles closely wedged together, and the 

 genital plates of P. atrata with its four distant, large secondary tubercles 

 (Pis. 25, fig. 8; 26, figs. 1,3). 



On Plate 11 has been figured a series of spines of a specimen measuring 

 82 mm. in diameter selected to represent the great difference and varia- 

 tion among the primary radioles of the ambital and abactinal part of the test 

 as well as of the spines of the actinal region and of the miliary spines 

 hidden by the pavement of primary radioles of the abactinal system (Pis. 11, 

 fig. 1; 46-48). Figure 1 also shows the great development of the power- 

 ful muscular system which holds the radioles in place upon their corresponding 

 primary tubercles. Figures 2-10 of Plate 11 show the shape of the radioles of 

 the plates of the buccal membrane of the actinal system and of the first plates 

 of the odd anterior ambulacrum and anterior interambulacrum. These radioles 

 are all finely striated, somewhat club-shaped, slightly flattened towards the tip, 

 with an indistinct milled ring and a deep socket at the base of the radioles. 



