22 TROPICAL PACIFIC ECHINI. 
of Podophora pedifera measuring 55 mm. in diameter (PI. 12, fig. 7) with the 
same interambulacrum of Podophora atrata of a somewhat smaller specimen 
(Pl. 24, fig. 4) will show at once the great difference there is in the tuberculation 
of the two species of the genus Podophora. Their development results in the 
one case (P. pedifera) in a specimen of twelve and thirteen plates to four 
vertical rows, and in the other (P. atrata) to three vertical rows on each inter- 
ambulacral plate in a specimen of twelve and twelve interambulacral plates. 
The narrowness of the actinal part of the interambulacral zone is very marked 
as contrasted to P. atrata (Pl. 12, fig. 1),as well as the extension of the lip of 
the gill slits to the fourth plate from the actinostome separating the two outer 
vertical zones from the inner ones. The difference in outline of the scrobic- 
ular areas and the mammary boss of this species (Pl. 12, figs. 2-6) is as marked 
as in P. atrata. There are fewer miliaries than in P. atrata, owing to the 
comparatively larger size of the primary tubercles which leave less space 
between the scrobicular areas of the primary tubercles for their development. 
I have given figures on Plate 10 showing the order of development of the 
primary tubercles of some large specimens of P. pedifera measuring from 76 
to 87 mm. in diameter (PI. 10, figs. 1-6). We can trace in all the figures the 
radioles belonging to the two outer vertical rows, those of the primary row, 
and those adjacent to the median line. As will be seen on examining the 
figures, the position of the radioles shows the greatest regularity in the growth 
of the eight vertical rows of the interambulacral system of P. pedifera just as 
they did in the growth of the six vertical rows of the interambulacral zone of 
P. atrata. Seen from the interior of the test the plates of the right anterior 
interambulacra adjoining the actinal system show what seems to be the ir- 
regular remnant of the primordial plate (Pl. 14, fig. 8). Such a plate does 
not exist in the odd posterior interambulacrum (Pl. 14, fig. 9). 
In a section of P. pedifera (Pl. 13, fig. 1) the outline of the test at the 
ambitus is seen to be pentagonal, the point of the pentagon running across 
the median line of the odd posterior interambulacrum. The pentagon of the 
actinal system runs in the opposite direction. The gradual narrowing up of 
the interambulacral plates at the ambitus is shown from the same figure, as 
well as the tufts of miliary spines rising at the base of flattened primary spines 
in both the ambulacral and interambulacral areas. 
The abactinal part of the ambulacral system consists of simple pairs of 
pores, the secondary plates of which extend about two thirds across the pri- 
mary plates (Pl. 13, fig. 8). The second plate has eight pairs of pores, the upper 
