26 TROPICAL PACIFIC ECHINI. 
mens (Pl. 28, figs. 3, 4,7 a, 8 a) but do not compare in length with those of 
P. pedifera (Pl. 15, figs. 8, 4, 7 a, 9a). The compass is distinctly jointed 
in P. pedifera and in the young of P. atrata (Pls. 15, fig. 13; 27, fig. 14); in 
the old of atrata there is no trace of any suture. The differences in the braces 
are very slight. 
In Colobocentrotus Stimpsoni, the pyramids are broader at the summit (PI. 
44, figs. 3, 4) than the pyramids of the other species; the corrugations are 
coarser (PI. 44, figs. 4, 5, 8 a, 9); the apophysal foramen is wider than that of 
P. pedifera, and the hooks are intermediate in length between those of P. pe- 
difera and P. atrata. |The compass is jointed (Pl. 44, figs. 12 a, 3). 
The auricles of a small specimen of P. atrata, 8 mm. in diameter, are not 
united (Pl. 26, figs. 9, 10); they touch in a specimen of 14 mm. (Pl. 26, 
fig. 12) and are connected in a specimen of 45 mm. (Pl. 26, fig. 13). Ina 
large specimen of P. pedifera 82 mm. in diameter they are closely connected 
(Pl. 14, figs. 5, 6); the same is the case in a specimen of Colobocentrotus 
Stimpsoni of 88 mm. in diameter (Pl. 43, figs. 4, 5). 
The pores close to the actinal system show how the ambulacral plates grad- 
ually increase in number and become resorbed as they move into the base of 
the auricles both from the actinal and the abactinal sides (see Pls. 14, figs. 5, 
6; 26, figs. 12, 13; 43, figs. 4,5; compare also Pl. 26, fig. 9 with Pl. 26, figs. 12, 
13). The youngest poriferous plates are still seen on Pl. 14, fig. 6, and Pl. 43, 
fig. 4; somewhat older plates are shown on Pl. 14, fig. 7, which have been 
resorbed by the auricle (Pls. 14, fig. 5; 26, fig. 13); the upper part of the 
auricle in fig. 7, Pl. 14, is cut off. 
THE SPICULES OF THE PEDICELS OF COLOBOCENTROTUS AND PODOPHORA. 
Pls. 16; 45. 
A large number of pedicels of Colobocentrotus and Podophora have been 
examined (Pls. 16, 45). They all agreed in having a disk made up of four 
basal plates; the periphery of the plates is angularly lobed. The number of 
lobes varies from seven to thirteen. In Colobocentrotus Stimpsoni (PI. 45, 
figs. 1, 2) the plates have from twelve to thirteen lobes, while in the other 
species of the genus, Colob. Mertensii, the basal disks have usually not more 
than seven to nine lobes. See the figures of the disk of an actinal pedicel 
of P. pedifera (Pl. 16, figs. 1-7), of P. atrata (Pl. 16, figs. 9-11, 13) and of 
Colob. Mertensii (Pl. 45, fig. 7). At the base of the basal disk on the lower 
