THE GENUS COLOBOCENTROTUS. 17 
The denuded test of Pl. 24, fig. 4, corresponds nearly to that of Pl. 22, fig. 
10, covered with spines; that of Pl. 22, fig. 6, to a stage intermediate between 
Pl. 24, figs. 2 and 3. 
In a small specimen of Podophora atrata the actinal plates of the odd inter- 
ambulacral zone did not show any trace of the primordial plate (Pl. 26, fig. 77). 
The passage of the regular poriferous ares into the laterally expanded porif- 
erous field on the actinal side of a specimen 45 mm. in diameter is shown on 
Pl. 25, fig. 10, where the upper plate is on the abactinal surface and the lower 
plate on the actinal side of the test, the former having an are of nine porifer- 
ous plates, the other of eleven pairs of pores arranged diagonally across the 
outer part of the plate, which is considerably larger than the plate above it 
(Pl. 25, fig. 70). The lower poriferous pair is pushed towards the median 
ambulacral line. 
In a younger specimen of 8 mm. in diameter (PI. 25, fig. 7), taking the same 
plates of the odd ambulacral zone partly on the actinal side and partly above 
the ambitus, we find on the abactinal side the same arc of nine poriferous plates 
arranged round the base of the primary ambulacral tubercle, and on the 
actinal side thirteen pairs of pores arranged diagonally across the outer side 
of the more elongated actinal ambulacral plates preceding it. 
The arrangement of the arcs of poriferous plates on the abactinal side of 
the test is seen on Pls. 20, figs. 4, 6; 21, figs. 2, 3; 2, figs. 3, 4, 5, and the 
subsequently developed actinal poriferous field on Pls. 20, fig. 2; 21, fig. 1; 
25, figs. 2, 6, 9. There are many miliaries in the poriferous field (Pl. 25, 
fig. 9) in marked contrast to their absence in P. pedifera. 
A figure of P. atrata from the actinal side showing the arrangement of the 
poriferous field has been given by Lovén, (Btudes Pl. XVIII, fig. 158). 
The arrangement of the abactinal part of the ambulacral system of Podo- 
phora atrata is somewhat different from that of P. pedifera. The youngest 
plate is at once followed by a plate cut in two by an upper poriferous plate 
extending across the ambulacral plate and followed by nine poriferous demi- 
plates; these extend from the edge of the interambulacral zone half way to the 
median ambulacral line (see Pl. 26, figs. 15, 16). 
In an interior view of the abactinal part of the test (PI. 25, fig. 5) the ar- 
rangement and number of the ambulacral plates are clearly drawn and the 
succession of the large and small ambulacral plates well indicated. In a part 
of the odd anterior ambulacrum of a larger specimen, 45 mm. in diameter, 
we can follow close to the ambitus the beginning of the widening of the 
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