THE GENUS COLOBOCENTROTUS. 17 



The denuded test of PI. 24, fig. 4, corresponds nearly to that of PI. 22, fig. 

 10, covered with spines ; that of PI. 22, fig. 6, to a stage intermediate between 

 PI. 24, figs. 2 and 3. 



In a small specimen of Podophora atraia the actinal plates of the odd inter- 

 ambulacral zone did not show any trace of the primordial plate (PL 26, fig. ii). 



The passage of the regular poriferous arcs 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 arc 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. 10). The lower poriferous pair is pushed towards the median 

 ambulacral line. 



In a younger specimen of 8 mm. in diameter (PL 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 Pis. 20, figs, l^, 6; 21, figs. £, 3; 25, figs. 3, 4., 5, and the 

 subsequently developed actinal poriferous field on Pis. 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 actmal side showing the arrangement of the 

 poriferous field has been given by Loven, (Etudes 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 (PL 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 



