THE GENUS COLOBOCENTROTUS. ‘i 
horse-shoe shape and at the ambitus they are beginning to be crowded out of 
place to form the actinal poriferous field. PI. 46, fig. 10 is a pair of pores from 
the poriferous field, fig. 11 a pair of pores at the ambitus, and fig. 12 one of 
the pores of the twelfth ambulacral plate, counting from the actinal system. 
CoLosocentrotus Brandt (A. Ag. emend.) 
Colobocentrotus Mertensii, Brandt. 
Pls. 2, figs. 8-13 ; 3°, figs. 9-11; 30; 32, figs. 5-8; 35; 36; 39, 
figs. 3-4 ; 44; 46, figs. 6-10. 
Colobocentrotus Mertensii differs from Colob. Stimpsoni in having a more 
circular outline (Pl. 36, figs. 1, 2) and being less depressed (Pl. 36, fig. 3). 
It has a proportionally larger actinal system (Pl. 36, fig. 1). The actinal 
surface is more sunken than that of Colob. Stimpson. The ambital edge 
both of the ambulacral and interambulacral zones carries much larger prim- 
ary tubercles (Pl. 36, figs. 1, 3) than those of Colob. Stimpsoni. The actinal 
lips are broad; the actinal plates of the interambulacral areas are narrower 
than those of Colob. Stimpson’. The actinal interambulacral zone carries four 
irregular, vertical rows of most distinct small secondaries larger than in the 
corresponding zones of Colob. Stimpsont. 
The poriferous fields are broad, slightly sunken ; the median line of the small 
distant ambulacral primaries consists of two vertical rows also larger than 
those of Colob. Stimpson. The poriferous field is studded with minute miliaries 
and small secondaries. 
The arrangement of the miliaries on the buccal plates and of the cluster of 
narrow crescent-shaped plates of the actinal system (Pl. 36, fig. 1), does not 
differ materially from that of Colob. Stimpsoni (Pl. 38, fig. 1). The five pairs 
of poriferous buccal plates are somewhat larger in Colob. Stimpsoni than in 
Colob. Mertensii. At the ambitus there are two rows of eight large primary 
interambulacral tubercles. These rapidly taper to four rows of small tubercles 
towards the actinal system; on the abactinal side the primary interambu- 
lacral tubercles diminish gradually to the second or third plate from the am- 
bitus and then remain of nearly uniform size, diminishing most gradually 
towards the abactinal system. At the fourth plate from the apex there are 
but six primary tubercles to each interambulacral plate, arranged in two hori- 
zontal rows; while nearer the ambitus there are eight or ten similarly arranged. 
The intertubercular space is filled with miliaries (Pl. 36, figs. 2, 3). 
