8 TKOPICAL PACIFIC ECHINI. 



In the ambulacra! zone there are four vertical rows of large primary 

 tubercles at the ambitus. These gradually diminish in size towards the abao- 

 tinal system. On each ambulacral plate there are three primary tubercles, 

 two, one above the other, along the rnedian ambulacral line, the other on the 

 distal side of the plate, surrounded by the poriferous arc. This consists of 

 seven pairs of pores, while there are eight in Colob. Stimpsoni. The intertu- 

 bercular space is filled with miliaries. 



The abactinal system is more raised than in Colob. Stimpsoni, and it is 

 covered by a coarser tuberculation of small secondaries (PI. 36, fig. 2), The 

 anal system is deeply sunken ; the genital pores are slightly larger than those 

 of Cohb. Stimpsoni. There are from ten to twelve closely packed tubercles 

 on the genital plates, and three on the ocular plates. The madreporic body 

 occupies the two sides of the distal part of the right anterior genital ; the 

 proximal part adjoining the anal system carries a few small secondaries and 

 miliaries. 



Colohoceiitrotus Mertensii, when covered with spines (PI. 35) is of a 

 darker ash color than Colob. Stimpsoni. The actinal spines of the former, 

 corresponding to the longer small primary tubercles of that region, are longer 

 and larger (PL 35, fig. 1) than those of Colob. Stimpsoni (PI. 33, fig. 1), and 

 the large, flat, truncated radicles are more numerous at the ambitus than in 

 Colob. Stimpsoni. 



In Colob. Mertensii, the primary radioles of the abactinal side of the test 



are larger than those of Cohb. Stimpsoni (compare PI. 35, figs. 2, 3 with PI. 33, 



/i / 2.-3/ fig-"'^) and are also more distant, giving a freer circulation of water round 



/ / the spines to the miliaries and pedicellarioe of the intertubercular areas. The 



small radioles covering the abactinal system (PI. 35, fig. 2) are more pointed 



than those of the abactinal system of Colob. Stimpsoni (PI. 33, fig. S). 



The order of appearance of the vertical rows of primary tubercles of Colob. 

 Merteiuii is shown on PI. 39, figs. 1, 2, which represent the arrangement of 

 the radioles (fig. 1) in the right posterior ambulacrum, with the right poste- 

 rior interambulacrum and a part of the right anterior ambulacrum. Fig. 2 

 shows the arrangement of the radioles of the odd posterior interambulacrum, 

 with the bivium and half of the right posterior ambulacrum. 



