30 TROPICAL PACIFIC ECHINI. 



The valves of the tridentate pedicellarite of Colob. Stimpsoni vary in length 

 from .78 mm. to .88 mm. ; those of Coloh. Mertensii from .41 to .81 mm. 



The CySTACANTHS and SPHiERIDIA OF COLOBOCENTROTUS AND PODOPHORA. 



Pis. 1-3 ; 31 ; 32. 



A comparison of the sphaeridia of the species of Colobocentrotus and Podo- 

 phora shows at once how variable they are and how misleading their charac- 

 ters seem as guides to the affinities of the species of the genus. From a first 

 glance at the figures of Pis. 31 and 32, we would arrange on one side the more 

 or less cylindrical sphaeridia of P. jfedifera (PI. 31, figs, i-^), and the three 

 other species with globular sphaeridia on the other. Yet no one would attempt 

 to say that P. atrata is more closely allied to Colob. Stimpsoni and Colob. 

 Mertensii than to P. pedifera, as would be legitimate from the sphaeridia 

 alone. 



PI. 13, fig. 5, shows a line of sphaeridia on the actinal part of the right 

 ambulacral zone. The great variation which exists in the sphisridia of P. 

 Ij^^jLoUa-O^ I -e4fmtu is well seen on comparing the figures of sphaeridia taken from different 

 ' / parts of the same individual (measuring 8^ mm. in diameter, including the ^V 



spines), viz., the left sphaerid of PI. 31, fig. 1, the left of fig. 2, the middle of ' 

 fig. 3, and the right of fig. J!^, taken respectively from the right posterior am- 

 bulacrum, the odd anterior ambulacrum, the left anterior ambulacrum, and the 

 left posterior ambulacrum. They vary in length from .19 mm. to .34 mm., 

 and in outline are cylindrical, elliptical, or club-shaped. 



In P. «('/•«/« the variation in shape is not as marked (PI. 31, figs. 5-S), 



yet there are such extremes in shape as the right sphrerid of PI. 31, 



fig. 6, and the right sphaerid of PI. 31, fig. 7, the one measuring .2(j( mm., ^y 



L I the other .23 mm. in length ; both were taken from the same specimen of 



r/Y/eS mm. in diameter including the spines ; the one from the odd anterior ambu- 



' lacrum, the other from the left posterior ambulacrum. 



One might think, on examining the figures of Plate 32, that the sphferidia 

 of Coloh. Stimpsoni and Colob. Mertensii differed but little if the comparison is 

 made between such sphaeridia as PI. 32, figs. 1, ^, and PL 32, fig. 7, and the 

 central figure of fig. 5. When we come to compare such extremes of Colob. 

 Stimjjsoni as PI. 32, fig. 1, with the extremes of Cobb. Mertensii (PI. 32, fig. 6), 

 the differences are most striking ; while a comparison of the left figure of 

 PI. 32, fig. 7 {Coloh. Mertensii) with the left figure of PI. 32, fig. 8 {Colob. 



