30 TROPICAL PACIFIC ECHINI. 
The valves of the tridentate pedicellarizxe of Colob. Stimpsoni vary in length 
from .78 mm. to .88 mm.; those of Colob. Mertensii from .41 to .81 mm. 
Tue CYSTACANTHS AND SPHZRIDIA OF COLOBOCENTROTUS AND PopDOPHORA. 
Pls. 1-3; 31; 32. 
A comparison of the spheridia of the species of Colobocentrotus and Podo- 
phora shows at once how variable they are and how misleading their charac- 
ters seem as guides to the affinities of the species of the genus. From a first 
glance at the figures of Pls. 31 and 32, we would arrange on one side the more 
or less cylindrical spheeridia of P. pedifera (Pl. 31, figs. 1-4), and the three 
other species with globular sphzeridia on the other. Yet no one would attempt 
to say that P. atrata is more closely allied to Colob. Stimpsoni and Colob. 
Mertensti than to P. pedifera, as would be legitimate from the spheridia 
alone. ; 
P]. 13, fig. 5, shows a line of spheridia on the actinal part of the righ 
ambulacral zone. The great variation which exists in the spheridia of P. 
atrata is well seen on comparing the figures of spheeridia taken from different 
parts of the same individual (measuring 82 mm. in diameter, including the 
spines), viz., the left sphzerid of Pl. 31, fig. 1, the left of fig. 2, the middle of 
fig. 3, and the right of fig. 4, taken respectively from the right posterior am- 
bulacrum, the odd anterior ambulacrum, the left anterior ambulacrum, and the 
left posterior ambulacrum. They vary in length from .19 mm. to .34 mm., 
and in outline are cylindrical, elliptical, or club-shaped. 
In P. atrata the variation in shape is not as marked (Pl. 31, figs. 5-8), 
yet there are- such extremes in shape as the right spherid of Pl. 31, 
fig. 6, and the right spherid of Pl. 31, fig. 7, the one measuring .20 mm., 
the other .25 mm. in length; both were taken from the same specimen of 
65 mm. in diameter including the spines; the one from the odd anterior ambu- 
lacrum, the other from the left posterior ambulacrum. 
One might think, on examining the figures of Plate 32, that the spheridia 
of Colob. Stimpsoni and Colob. Mertensii differed but little if the comparison is 
made between such spheeridia as Pl. 32, figs. 1, 4, and Pl. 32, fig. 7, and the 
central figure of fig. 5. When we come to compare such extremes of Colod. 
Stimpsoni as Pl. 32, fig. 1, with the extremes of Colob. Mertensii (Pl. 32, fig. 6), 
the differences are most striking; while a comparison of the left figure of 
Pl. 32, fig. 7 (Colob. Mertensii) with the left figure of Pl. 32, fig. 3 (Colod. 
