THE GENUS COLOBOCENTROTUS. 29 
edge of Pl. 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 (Pl. 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 (Pl. 18, figs. 1-3). It would be difficult to say which 
of these pedicellariz are typical of P. atrata ; as in P. pedifera the valves of 
the triphyllous pedicellarize (Pls. 18, figs. 10-12; 19, figs. 14-16) vary greatly 
in outline and in ornamentation. The valves figured were taken from two 
specimens; they vary in length from .1]1 mm. to .16 mm. 
Compared with the globiferous pedicellarie of P. pedifera, though they 
vary greatly (Pls. 4, figs. 18-22 ; 5, figs. 10-13), those of P. atrata (PI. 18, figs. 
17, 18) do not show any important differences. 
One of the ophicephalous pedicellariz 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 pedicellarie of P. pedifera, 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- 
cellariz of Colob. Stimpsoni (Pl. 29, figs. 1-6, 10-13, 16), varying in length 
from .36 mm. to .51 mm., nor those of Colob. Mertensii (Pl. 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. pedifera or P. atrata. It is quite possible 
to select ophicephalous pedicellarie of P. pedifera and Colob. Stimpsoni 
which are very similar, and which it would be difficult to distinguish 
(see Pl. 6, fig. 10 and Pl. 29, fig. 13). The valves of the triphyllous 
pedicellarize of Colob. Stimpsoni (Pl. 29, figs. 14-15) appear to be somewhat 
more slender than those of Colob. Mertensii (Pl. 30, figs. 10-11, 16), and the 
foramina of the valves of the triphyllous pedicellarie of these two species are 
perhaps, on the whole, smaller than those of P. pedifera and P. atrata. 
The tridentate pedicellarize of Colob. Stimpsoni (PI. 29, figs. 7-9) and those 
of Colob. Mertensii (Pl. 30, figs. 3-8) resemble more the general pattern of 
those of P. pedifera, 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. 
