THE GENUS COLOBOCENTROTUS. 9 



Colobocentrotus Stimpsoni A. Ag.^ '^^ ^7-— j/Uy 



Pis. 2, figs, lt-1 ; 29 ; 32, figs. 1-k ; 33 ; 34 ; 37-39, figs.^-^; ^4/ 



40-45, figs. 1-5. ' 



A specimen of Coloh. Stimpsoni, measuring 65 mm. when denuded, shows 

 its elliptical outline (Pis. 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 (Pis. 34, figs. 1, 3; 38, figs. 1, 3). A profile view 

 (Pis. 34, fig. 3 ; 38, fig. 3) shows the nearly flat actinal side and low, subconi- 

 cal arched test. A view from the actinal side (Pis. 34, fig. 1 ; 38, fig. l) 

 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 (PI. 42, fig. 1). 



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 PI. 42, fig. 1), this is also seen in the compression of one of the ambulacral 

 ambital plates (PI. 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 



