26 TROPICAL PACIFIC ECHINI. 



mens (PI. 28, figs. 3, ^,7 a, 8 a) but do not compare in length with those of 

 P. pedifera (PL 15, figs. 3, 4, 7 a, 9 a). The compass is distinctly jointed 

 in P. pedifera and in the young of P. atrata (Pis. 15, fig. 13 ; 27, fig. U) ; in 

 the old of atrata there is no trace of any suture. The differences in the braces 

 are very slight. 



In Colobocentrotiis Stimp)Soni, the pyramids are broader at the summit (PI. 

 44, figs. 3, 4) than the pyramids of the other species ; the corrugations are 

 coarser (PI. 44, figs. 4, 5, 8 a, 9) ; the apophysal foramen is wider than that of 

 P. pedifera, and the hooks are intermediate in length between those of P. j^e- 

 difera and P. atrata. The compass is jointed (PI. 44, figs. 12 a, b). 



The auricles of a small specimen of P. atrata, 8 mm. in diameter, are not 

 united (PL 26, figs. 9, 10) ; they touch in a specimen of 14 mm. (PL 26, 

 fig. 12) and are connected in a specimen of 45 mm. (PL 26, fig. 13). In a 

 large specimen of P. j^edifera 82 mm. in diameter they are closely connected 

 (PL 14, figs. 5, 6); the same is the case in a specimen of Colohocentrotus 

 Sti^npsoni of 88 mm. in diameter (PL 43, figs. 4, 5). 



The pores close to the actinal system show how the ambulacral plates grad- 

 ually increase in number and become resorbed as they move into the base of 

 the auricles both from the actinal and the abactinal sides (see Pis. 14, figs. 5, 

 6; 26, figs. 12, 13; 43, figs. 4, 5; compare also PL 26, fig. 9 with PL 26, figs. 12, 

 13). The youngest poriferous plates are still seen on PL 14, fig. 6, and PL 43, 

 fig. 4; somewhat older plates are shown on PL 14, fig. 7, which have been 

 resorbed by the auricle (Pis. 14, fig. 5; 26, fig. 13); the upper part of the 

 auricle in fig. 7, PL 14, is cut off. 



TuE Spicules of the Pedicels of Colobocentrotus and Podophora. 



Pis. 16 ; 45. 



A large number of pedicels of Colobocentrotus and Podophora have been 

 examined (Pis. 16, 45). They all agreed in having a disk made up of four 

 basal plates ; the periphery of the plates is angularly lobed. The number of 

 lobes varies from seven to thirteen. In Colohocentrotus Stimpsoni (PL 45, 

 figs. 1, 2) the plates have from twelve to thirteen lobes, while in the other 

 species of the genus, Colob. Mertensii, the basal disks have usually not more 

 than seven to nine lobes. See the figures of the disk of an actinal pedicel 

 of P. pedifera (PL 16, figs. 1-7), of P. atrata (PL 16, figs. 9-11, 13) and of 

 Coloh. Mertensii (PI. 45, fig. 7). At the base of the basal disk on the lower 



