116 PANAMIC DEEP SEA ECHINI. 



there we also find small bare spaces between the sutures of the anal plates. 

 The anal opening is large, a funnel-like projection of a thick skin, the 

 edges strengthened by long narrow slender plates, followed by an irregular 

 second row of larger elongated plates, and then by two to three rows of 

 most irregularly arranged polygonal plates covered with distinct small 

 secondaries and miliaries. 



The madreporite occupies four plates. It is difficult to say if the two 

 outer plates belong to the genital plate, or are anal plates which have forced 

 their way between the adjoining oculars and the right anterior genital. 

 The plates of the genital ring are comparatively bare. The oculars carry 

 but one or two small secondaries with four or five small miliaries. On the 

 component plates of the genitals the tuberculation is somewhat closer, but a 

 great part of the plates is also bare. 



The interambulacral plates become quite early separated by a bare 

 horizontal belt free from calcareous tissue. In the odd posterior interambu- 

 lacrum (PI. 52, fig. /) the first, second, and third of the abactinal interambu- 

 lacral plates are connected along the whole length of the horizontal 

 suture, the third and fourth plates are slightly separated by a bare space, 

 while between the next plates the calcified part of plates occupies already 

 the proportions of the interambulacral area of older plates near the 

 ambitus. 



A part of the abactinal system, 1 when seen from the interior of the test 

 (PI. 52, fig. 2), shows the extraordinary splitting up of the calcareous plates 

 into ossicles almost like those of the abactinal surface of starfishes, not only 

 of the interambulacral but to a certain extent of the ambulacra! area, as also 

 of the anal plates, of the genitals and oculars. The extent to which the 

 interambulacral ossicles overlap laterally with the ambulacra] ones is well 

 seen on Plate 52, figs. 3, 4, taken not far from the ocular plate; fig. J 

 showing this overlapping as seen from the exterior, and fig. .', from the 

 interior. The dermal membrane separating longitudinally the interambu- 

 lacral and ambulacral plates is shown in Plate 52, fig. .?. 



The interambulacral plates are made up of a series of short joints, from 

 five to six in the larger plates, and the confusion arising from this breaking 

 up and the forcing of the anal plates through the median line of the interam- 

 bulacral system is well seen in Plate 52, fig. ;?, in the spare between the 

 oculars and genitals, — a confusion of plates which is even worse than when 



1 The right posterior ocular and genital. 



