18 TROPICAL PACIFIC ECHINI. 



ambulacral plates and the flattening of the smaller intercalated plate (PL 26, 

 fig. H). In Podophora atrata there are larger miliaries than in P. pedifera 

 along the median ambulacral line (PI. 21, fig. 3). 



In P. atrata the pavement of radioles covering the abactinal system is made 

 np of more irregularly shaped radioles than in. P. jndifera (PI. 26, figs. 2, ^). 

 Both the genital and ocular plates are more elongate than those of P. pedi- 

 fera, and the genital plates carry from two to four small secondaries with a 

 few small miliaries irregularly scattered. The ocular plates, as in P . pedifera, 

 also carry one small secondary tubercle, with five to ten small miliaries (PI. 

 26, fig. 3). In all the species both of Colobocentrotus and Podophora the 

 ocular plates of the bivium are nearer the anal system than those of the 

 trivium. The right anterior genital plate is nearly covered" by the madrepo- 

 rite, and carries a few small secondaries along the median line with two or 

 three clusters of miliaries. 



In a smaller specimen (PI. 25, fig. 3) there are few miliaries on the abac- 

 tinal system (PI. 26, fig. i), with three to four secondary tubercles of uniform 

 size on the genitals, and few miliaries. Seen from the interior (PI. 25, fig. 5) 

 the sutures of the abactinal system are well shown. The madreporite is a 

 narrow band extending across the anterior extremity of the right anterior 

 genital (Pis. 26, fig. 1; 25, fig. 3). 



The genital pores are proportionally smaller in P. atrata (Pi. 26, fig. 3) 

 than in P. pedifera, and are placed as in that species at the very tip of the 

 genital plates. In the young specimen (PI. 26, fig. i) there is as yet no trace 

 of genital pores, but the ocular pores are well developed. 



PI. 26, fig. 5 shows the anal system of a specimen measuring 8 mm. in 

 diameter (PI. 26, fig. i), with eight plates covering the whole anal system. 

 In a somewhat larger specimen the number of plates has increased, and they 

 are not as regularly arranged. In a still larger specimen there are two rows 

 of plates, the outer one being composed of large triangular ones (PI. 26, fig. 7), 

 and in a specimen of 45 mm. in diameter (PI. 26, fig. 8) we have a magnified 

 view of the anal plates of PI. 26, fig. 3. 



The principal types of radioles of Podophora atrata are represented on PI. 23. 



Fig. i is a small spine from the actinal membrane near tlie teeth. 



Figs. 2-5 are from the odd ambulacrum and the left anterior interambula- 

 crum near the actinal system. 



Figs. 6-8 are views of a slightly flattened primary radiole from the left 

 posterior interambulacrum from the actinal side of the ambitus. 



