8 TROPICAL PACIFIC ECHINI. 
In the ambulacral zone there are four vertical rows of large primary 
tubercles at the ambitus. These gradually diminish in size towards the abac- 
tinal system. On each ambulacral plate there are three primary tubercles, 
two, one above the other, along the median ambulacral line, the other on the . 
distal side of the plate, surrounded by the poriferous arc. This consists of 
seven pairs of pores, while there are eight in Colob. Stimpsoni. The intertu- 
bercular space is filled with miliaries. 
The abactinal system is more raised than in Colob. Stimpsoni, and it is 
covered by a coarser tuberculation of small secondaries (Pl. 36, fig. 2). The 
anal system is deeply sunken; the genital pores are slightly larger than those 
of Colob. Stimpson. There are from ten to twelve closely packed tubercles 
on the genital plates, and three on the ocular plates. The madreporic body 
occupies the two sides of the distal part of the right anterior genital; the 
proximal part adjoining the anal system carries a few small secondaries and 
miliaries. 
Colobocentrotus Mertensti, when covered with spines (Pl. 35) is of a 
darker ash color than Colob. Stimpsoni. The actinal spines of the former, 
corresponding to the longer small primary tubercles of that region, are longer 
and larger (Pl. 35, fig. 1) than those of Colob. Stimpsoni (Pl. 33, fig. 1), and 
the large, flat, truncated radioles are more numerous at the ambitus than in 
Colob. Stimpsoni. 
In Colob. Mertensii, the primary radioles of the abactinal side of the test 
are larger than those of Colob. Stimpsoni (compare PI. 35, figs. 2, 3 with Pl. 33, 
fig. 22) and are also more distant, giving a freer circulation of water round 
the spines to the miliaries and pedicellarie of the intertubercular areas. The 
small radioles covering the abactinal system (Pl. 35, fig. 2) are more pointed 
than those of the abactinal system of Colob. Stimpsoni (Pl. 33, fig. 2). 
The order of appearance of the vertical rows of primary tubercles of Codd. 
Mertensii is shown on Pl. 39, figs. 1, 2, which represent the arrangement of 
the radioles (fig. 2) in the right posterior ambulacrum, with the right poste- - 
rior interambulacrum and a part of the right anterior ambulacrum. Fig. 2 
shows the arrangement of the radioles of the odd posterior interambulacrum, 
with the bivium and half of the right posterior ambulacrum. 
