4 TROPICAL PACIFIC ECHINI. 



Podophora atrata Agass. 



Length, denuded, 45 mm. 

 Width, denuded, 38 mm. 

 Height, denuded, 18 mm. 

 Pairs of pores in each ambulacral plate 9, 10, near the ambitus on the 

 abactinal side. 



Colobocentrotus Stimpsoni A. Ag. 



Length, denuded, 64 mm. 

 Width> denuded, 53 mm. 

 Height, denuded, 24 mm. 

 Pairs of pores in each ambulacral plate 7, 8, near the ambitus on the 

 abactinal side. 



Colobocentrotus Mertensii Brandt. 



Length, denuded, 54 mm. 

 Width, denuded, 48 mm. 

 Height, denuded, 22 mm. 

 Pairs of pores in each ambulacral plate 7, 8, near the ambitus on the 

 abactinal side. 



The outline at the ambitus of Podophora pedifera (Fig. a ; PI. 13, fig. 1) 

 and of Podophora atrata (Fig. h; PL 25, fig. 5) Ls pentagonal in both; the 

 actinal side of Podophora pedifera is strongly arched, while that of Podophora 

 atrata is nearly flat. The ambitus of Colob. Stimp)som is ellipsoidal (fig. d), in 

 Colob. Mertensii it is almost circular (fig. c). This dissimilarity can scarcely 

 be due to age, as the difference in length of the two specimens is only 10 mm. 

 The number of tubercles in Cohb. Mertensii is less than in Colob. Stimpsoni. 

 In Mertensii those in the interambulacrum at the ambitus, on the actinal 

 side, are somewhat larger ; hence that side is more concave. 



Photography is often, misleading when reproducing specimens of natural 

 size, some parts are apt to be somewhat out of focus. The profiles in Plates 

 8, fig. 3; 20, fig. 6; 34, fig. 3; and 36, fig. 3, do not fully correspond to the 

 diagrams (figs, a-d) which are correct. 



A comparison of the odd anterior ambulacrum of the two species of Podo- 

 phora (PI. 46, figs. 1, 5), and of Colobocentrotus Stimpsoni (PI. 46, fig. 9) 

 brings out strikingly their specific differences. In a specimen of Podophora 

 pedifera Agass. of 55 mm. in diameter the ambitus crosses the fifth and sixth 



