THE GENUS COLOBOCENTROTUS. 11 
Colobocentrotus Stimpson. These figures show how the distant openings for 
the pedicels slant across the thickness of the test to come together and form 
the pairs of pores. 
On Pl. 41, fig. 1, is given a figure of the left anterior interambulacrum of a 
specimen of Colob. Stimpsoni measuring with its spines 88 mm. in diameter 
with 14 and 15 coronal plates. At the ambitus on the abactinal side there 
are 12 rows of primary tubercles which form disconnected vertical rows ; at the 
tenth plate there are but five rows and at the third plate from the apex 
only three. The tubercles diminish but slightly in size from the ambitus 
towards the abactinal system and the intertubercular space is well filled with 
miliaries. Below the ambitus on the actinal side are found the largest of 
the primary tubercles forming two horizontal lines, the lower one of which is 
the largest, extending across the fifth and sixth plates from the actinal sys- 
tem. These tubercles carry the large flattened radioles truncated at the tip 
which form the ring of larger radioles round the base of the abactinal part of 
the test. 
Between the row of large tubercles and the actinal system the interambu- 
lacral plates on each side of the poriferous field become much narrower 
(Pls. 41, fig. 1; 38, fig. 2; 34, fig. 1). There are only four indistinct verti- 
cal rows of small primary tubercles. The rest of the plates is filled with 
still smaller primary tubercles and patches of miliaries irregularly arranged. 
The actinal lips are short but well marked. The lower part of fig. 1, Pl. 41, 
shows the distortion which takes place in the outline of the large primary 
tubercles below the ambitus owing to the great compression due to the forma- 
tion of: the sharp angle of the test at the ambitus. Fig. 3, Pl. 41, shows one 
of the most distorted of the primary tubercles on the fifth plate from the 
actinostome, the mammary boss, and the scrobicular area being pushed out of 
shape. Fig. 4 shows the same tubercle in profile. Fig. 2, Pl. 41, shows the 
more regular primary tubercles of the abactinal part of the test in which 
the boss and the scrobicular area are but slightly distorted, the boss and 
area being usually circular, Pl. 41, fig. 5. The grouping of the miliaries does 
not seem to follow any rule. PI. 41, fig. 6, shows those of one of the plates 
near the actinostome of the right posterior ambulacrum. 
In a specimen measuring 88 mm. in diameter, including the spines, it is pos- 
sible to trace the twelve vertical rows of the primary radioles of the interam- 
bulacral areas. In Pl. 39, fig. 3, the dotted lines show the twelve rows of the 
right posterior interambulacrum and the four vertical rows of the right poste- 
