THE GENUS COLOBOCENTROTUS. 29 



edge of PI. 19, figs. 1, 2, 3, 10, 11). In one case the serration is limited to a 

 few large teeth, in the other the edge is serrated for the whole length of the 

 blade. The size and arrangement of the foramina of the valve both of the 

 basal part and of the blade also varies greatly. In many of the valves 

 the base passes very gradually into the blade (PI. 19, figs. 7-10), in others the 

 junction is sharply marked (PI. 18, figs. 4, 5), and in others the outline of the 

 base is quite irregular (PI. 18, figs. 1-3). It would be difficult to say which 

 of these pedicellarifB are typical of P. atrata ; as in P. pedifera the valves of 

 the triphyllous pedicellarite (Pis. 18, figs. 10-12; 19, figs. 14.-I6) vary greatly 

 in outline and in ornamentation. The valves figured were taken from two 

 specimens; they vary in length from .11 mm. to .16 mm. 



Compared with the globiferous pedicellaritB of P. j^edifera, though they 

 vary greatly (Pis. 4, figs. 18-S2 ; 5, figs. 10-13), those of P. atrata (PI. 18, figs. 

 17, IS) do not show any important differences. 



One of the ophicephalous pedicellarioe of P. atrata is figured on PI. 18, figs. 

 7-9. The edge of the valves is more finely serrated than in the majority of 

 the ophicephalous pedicellarise of P. 2)edifera, but some of them are fully as 

 finely serrated as those of P. atrata. 



Owing to the great variation in the valves, neither the ophicephalous pedi- 

 cellarioe of Colob. Stimpsoni (PI. 29, figs. 1-6, 10-13, 16), varying in length 

 from .36 mm. to .51 mm., nor those of Coloh. Mertensii (PI. 30, figs. 1, 2, 9, 

 12-15), varying in length from .27 to .48 mm. show any character by 

 which it would be possible to distinguish these two species, or to dis- 

 tinguish them in turn from P. p)6difera or P. atrata. It is quite possible 

 to select ophicephalous pedicellariaa of P. pedifera and Coloh. Stimpsoni 

 which are very similar, and which it would be difficult to distinguish 

 (see PI. 6, fig. 10 and PI. 29, fig. 13). The valves of the triphyllous 

 pedicellarigB of Coloh. Stimpsoni (PI. 29, figs. H-15) appear to be somewhat 

 more slender than those of Coloh. Mei'tensii (PI. 30, figs. 10-11, 16), and the 

 foramina of the valves of the triphyllous pedicellariae of these two species are 

 perhaps, on the whole, smaller than those of P. pedifera and P. atrata. 



The tridentate pedicellarioe of Coloh. Stimpsoni (PL 29, figs. 7-9) and those 

 of Coloh. Mertensii (PI. 30, figs. 3-8) resemble more the general pattern of 

 those of P. j)^difera, but in this case also the difference in the ornamentation 

 of the valves, in the proportions of the base to the blade makes it im- 

 possible to designate any one figure as a typical tridentate valve of either 

 species. 



