94 PAXAMIC DEEP SEA ECHINI. 



In a specimen of 120 mm. (PI. 48, fig, 6) secondaries and miliaries 

 occupy the part of the plate between the poriferous zone and the interara- 

 bulacral edge, and the median vertical rows of primary tubercles are 

 prominent on the actinal side ; they continue somewhat smaller on the 

 abactinal side. Along the median space miliaries are arranged in irregular 

 horizontal lines, and on the actinal side the large primary plates are 

 covered with small secondaries. 



The buccal ambulacral plates are elongate, arranged in six rows (Pis. 41, 

 fig. 1 ; 48, fig. 6'), leaving but a narrow bare space in the interambulacral 

 area. The buccal plates are, from the earliest stage examined, all perforate 

 by a pair of pores, and carry a horizontal row of three to six small second- 

 aries on the actinal edge of the plate according to its length, with here 

 and there a miliary. 



The abactinal system of a specimen 120 mm. in diameter has undergone 

 great changes (PI. 39, fig. 7) from that of a specimen 75 mm. in diameter 

 (PI. 40, fig. 4). The abactinal median interambulacral area is occupied by 

 a triangular membraneous extension of the genital plate which reaches to 

 the third or fourth interambulacral plate from the summit, and in these 

 membraneous spaces are situated the genital openings. The calcareous part 

 of the genital plates is more or less elliptical. The left anterior genital 

 appears made up of the original genital and of an intercalated anal plate, 

 while the madreporic genital is made up of two primary genital plates 

 surrounded by an arc of eight anal plates, on seven of which the madre- 

 porite has encroached. The madreporic genital is the only one still partly 

 in contact with an ocular (the odd anterior ocular plate). The anal plates 

 have forced their way to such an extent between the oculars and genitals 

 that it is difficult to say where the interambulacral system ends or the 

 anal begins. PI. 40, fig. 5 shows the madreporic genital with the adjoin- 

 ing anal and other plates of the abactinal system as seen from the interior 

 of the same specimen. 



In another specimen somewhat larger (a fragment) of 130 mm. in 

 diameter (PI. 39, fig. i!) the genitals and oculars are all well separated by 

 anal plates which in some of the interambulacra have forced their way on 

 both sides of the median interambulacral space as far as the third inter- 

 ambulacral plate. The abactinal interambulacral plates are thus forced to 

 the sides, spread open at the summit to allow the further invasion of the 

 anal plates. This has changed the point of origin of the new interam- 



