12 TROPICAL PACIFIC ECHINI. 
rior ambulacrum. On fig. 4 of the same plate are shown the corresponding 
vertical rows of the right anterior ambulacrum and interambulacrum. In 
both the interambulacra two of the vertical rows along the median line are 
included by the inner and outer rows and do not extend either to the abactinal 
or the ambital region. 
The abactinal system when denuded shows that the genital plates are 
covered with closely packed primary tubercles only slightly smaller than those 
of the rest of the abactinal surface of the test above the ambitus. (Pls. 34, 
fig. 2; 38, fig. 2; 43, fig. 2.) There are in addition a few miliaries and small 
secondaries irregularly scattered between the larger tubercles. 
The madreporic genital is much larger than the other genitals ; the madre- 
porite occupies the greater of the two sides of the plate. The proximal part 
carries two primary tubercles and from four to five smaller ones as well as 
a few miliaries. The genital openings are large, slightly sunken ; the lateral 
posterior genitals are smaller than the odd posterior genital and the left ante- 
rior ones. (PI. 43, fig. 2.) 
The genital plates are heptagonal. The left anterior genital carries no less 
than sixteen primaries with a few miliaries and a couple of smaller primaries. 
(Pl. 41, fig. 7.) The other genitals carry from twelve to fourteen primaries 
with a few miliaries and secondaries. 
The ocular plates are pentagonal, they carry three primary tubercles with 
two or three small miliaries, the ocular pores are very small. The posterior 
oculars approach the anal system nearer than the anterior pair or the odd 
ocular. (Pl. 43, figs. 2, 3.) 
The anal system is irregularly circular and covered with three to four con- 
centric rows of irregularly shaped pointed and elliptical plates. (Pl. 43, figs. 
2, 3.) 
In a specimen of Colob. Mertensii measuring 88 mm., when seen from 
above the primary radioles covering the abactinal system are seen to be some- 
what smaller than those of the rest of the test (Pls. 33, fig. 2; 37, fig. 2; 43, 
fig. 1), with the exception of a few larger triangular and elliptical ones which 
bridge over the anal system (Pls. 40, figs. 30-32; 43, fig. 1). 
On Plates 33 and 37 are given views of a specimen of Colob. Mertensii, 
measuring 88 mm. in diameter. The ambital fringe of large truncated flat- 
tened primary spines carried by the larger tubercles on the actinal side of 
the ambitus and at the ambitus resemble slightly those of Podophora atrata ; 
they are comparatively shorter and narrower (Pls. 33, figs. 1, 2; 37, figs. 1, 2) 
