THE GENUS COLOBOCENTEOTUS. 7 



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 ambulacra! plate, counting from the actinal system. -iyCc-xro-uJ^ 



CoLOBOCENTROTUS Brandt (A. Ag. emend.) 



Colobocentrotus Mertensii, Brandt. 



Pis. 2, figs. 8-13 ; 3% figs. 9-11 ; 30 ; 32, figs. 5-8 ; 35 ; 36 ; 39, 

 /_7_/ figs.tM-; 44; 45, figs. G-IO. 



I Colobocentrotus Mertensii differs from Coloh. Stimpsoni in having a more 

 circular outline (PI. 36, figs. 1, 2) and being less depressed (PI. 36, fig. 3). 

 It has a proportionally larger actinal system (PI. 36, fig. 1). The actinal 

 surface is more sunken than that of Colob. Stimpsoni. The ambital edge 

 both of the ambulacra! and interambulacral zones carries much larger prim- 

 ary tubercles (PI. 36, figs. 1, 3) than those of Coloh. Stimpsoni. The actinal 

 lips are broad ; the actinal plates of the interambulacral areas are narrower 

 than those of Colob. Stimpsoni. The actinal interambulacral zone carries four 

 irregular, vertical rows of most distinct small secondaries larger than in the 

 corresponding zones of Colob. Stimpsoni. 



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. Stimpsoni. 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 (PI. 36, fig. 1), does not 

 differ materially from that of Colob. Stimpsoni (PI. 38, fig. 1). The five pairs 

 of poriferoas buccal plates are somewhat larger in Coloh. Stimpsoni than in 

 Coloh. 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 (PI. 36, figs. 2, 3). 



