THE GENUS COLOBOCENTROTUS. 9 
Colobocentrotus Stimpsoni A. Ag. 
Pls, 2, figs. 4-7 ; 29; 32, figs. 1-4; 33; 34; 37-89, figs. 4-5 ; 
40-45, figs. 1-65. 
A specimen of Colob. Stimpsoni, measuring 65 mm. when denuded, shows 
its elliptical outline (Pls. 34, figs. 1, 2; 38, figs. 1,2). The abactinal system 
is slightly raised; the anal system sunken; the poriferous zone is somewhat 
sunken, and the genital pores are deeply sunken. The mamelon of the abac- 
tinal primary tubercles is glossy, and the general appearance of this species, 
when seen from above, is that it is covered with a most uniform tuberculation 
diminishing but slightly in size towards the apical system. Immediately at 
the ambitus and on both sides of it are placed the horizontal rows of the 
largest primary tubercles (Pls. 34, figs. 1, 3; 38, figs. 1,3). A profile view 
(Pls. 34, fig. 3; 38, fig. 3) shows the nearly flat actinal side and low, subconi- 
eal arched test, A view from the actinal side (Pls. 34, fig. 1; 38, fig. 1) 
shows that the actinal surface of the test is flat, the ambital edge being 
slightly raised. The poriferous fields are wide, extend to the ambitus, and 
are separated by the comparatively narrow interambulacral zone. The two 
median vertical rows of small ambulacral tubercles extend to the actinostome. 
Two still smaller additional rows of secondary tubercles run across the porif- 
erous field about half way between the median line and the outer edge of the 
poriferous zone. 
Seen from the interior of the test the interambulacral plates show a well 
marked groove in the middle of the suture of adjoining plates which is not 
shown in the drawings owing to the absence of shading, and the poriferous 
zone is raised well above the general level of the remaining part of the 
ambulacral plates (Pl. 42, fig. 2). 
Owing to the flatness of the test of Colob. Stimpsoni and the sharp angle 
made by the ambitus between the abactinal and actinal sides the coronal 
plates at the ambitus are greatly compressed and the plates very narrow 
(see Pl. 42, fig. 2), this is also seen in the compression of one of the ambulacral 
ambital plates (Pl. 42, fig. 7) in which the sutures between the demi-plates 
carrying the pairs of pores are practically obliterated. Compare this with an 
abactinal plate, the tenth from the actinal system (PI. 42, figs. 5, 6), in which 
(fig. 7) the sutures of the demi-plates of the eight pairs of pores are sharply 
defined. 
In a view of the abactinal part of the ambulacral system, seen from the 
