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; Pl. 13, fig. 1) 
and of Podophora atrata (Fig. b; Pl. 25, fig. 5) is pentagonal in both; the 
actinal side of Podophora pedifera is strongly arched, while that of Podophora 
atrata is nearly flat. The ambitus of Colob. Stimpsoni 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 Colob. Mertensii is less than in Colob. Stimpson. 
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 (Pl. 46, figs. 1, 5), and of Colobocentrotus Stimpsoni (Pl. 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 
