6 TROPICAL PACIFIC ECHINI. 
with a few miliaries on each side of the median line separating the two porif- 
erous fields. The difference in appearance of the pores of the actinal edge of 
the poriferous field of the region at the ambitus and those of the abactinal side 
of the ambulacral system is shown on PI. 46, figs. 2, 3, 4. 
The odd anterior ambulacrum of a specimen of Podophora atrata meas- 
uring 46 mm. in diameter is at once distinguished from that of Podophora 
pedifera by the more regular outline of the vertical rows of primary tuber- 
cles. Both the mammary boss and scrobicular area are circular in outline 
(Pl. 46, fig. 5); they occupy a smaller are of the ambulacral plates, having a 
larger space free from miliaries along the median line of the ambulacral zone. 
There are here and there a few small secondary tubercles in the inner angles 
of the ambulacral plates. 
The vertical rows of primary tubercles extend as small tubercles to the 
edge of the actinal system, and numerous minute secondaries and miliaries are 
intercalated between the pairs of pores of the poriferous field. This is compar- 
atively narrower than that of Podophora pedifera, leaving a much broader 
actinal interambulacral zone to sepatate the poriferous fields of adjoining 
ambulacra (Pl. 24, fig. 7) than in Podophora pedifera. The change in the 
pairs of pores as we pass from the actinal edge to the ambitus and to one 
of the abactinal pairs of pores are seen in PI. 46, figs. 6, 7, 8. 
In a specimen of Colobocentrotus Stimpsoni measuring 65 mm. in longest 
diameter the primary tubercles above the ambitus are of nearly the same 
size with a circular mammary boss and scrobicular area (PI. 46, fig. 9), those 
at the ambitus being slightly larger and those on the actinal side of the ambi- 
tus decidedly larger. In these the mammary boss and the scrobicular area are 
somewhat elliptical. The abactinal primary tubercles form above the ambitus 
four slightly undulating vertical rows with small miliaries on both sides of 
the median line and miliaries between the outer and inner vertical rows. 
Below the ambitus there are four irregularly placed large primary tubercles 
with a somewhat smaller one, and below that two vertical rows of small, dis- 
tant secondaries on each side of the median line extending to the actinal 
system, with numerous miliaries and minute secondaries filling the spaces 
between the pairs of pores of the poriferous field. 
On the abactinal side of the test there are usually three large tubercles on 
each plate; near the abactinal system they are reduced to two or to one with 
a small secondary. The poriferous arcs occupy the outer part of the 
ambulacral plates. There are eight pairs of pores arranged in a very open 
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