184 PKOI". p. MARTIN BUNCAK OS THE 



zigzag close to its starting line from tlie ordinary level, and it is 

 very perceptible (with benzene, and sometimes without that excel- 

 lent distinguisher of sutures) that the median suture passes up the 

 median line of the ridge, but not in a right line, there being a 

 slight curving (fig. 2). 



The sides of the ridge on either side of its median suture 

 correspond with zones " a " and " b " of the interradium, and it 

 appears at first sight as if the side of the ridge corresponding 

 with, zone " &," for instance, was composed of a long plate the 

 circumferential angle of which extended beyond the ridge and 

 assisted to form part of the interradium, and that in the ridge 

 the zone " « " was composed of the whole of a plate *. 



But it is evident that in zone " h " the apparently long plate 

 is not a whole one, for there is a delicate suture which passes 

 from the convexity of the curve of the median suture, as seen 

 from within the test, and has an oblique upwards and sideways 

 course. T.his suture unites a small terminal plate (figs. 2 & 4). 

 There is no such plate in the other zone (a) . 



Both of these zones are limited by an ambulacro-interradial 

 suture. 



The direction of the sutural uuion of the terminal plate of zone 

 " h " with the plate immediately external or circumferential to it 

 is from the outer surface of the ridge inwards and downwards and 

 it terminates actinally. Hence this small plate comes to the under 

 or actinal surface of the test, and it carries there the smallest of 

 the interradial tubercles at the peristome. The plate external to 

 the small one has its sutural union also oblique and reaching the 

 actinal surface, and it is recognized thei'e as the second j^late of 

 the zone " 5 " which carries the first large tubercle (fig. 3). On the 

 other hand, the plate of zone " « " which forms tbe whole of one 

 side of the ridge reaches actinally and carries there the first 

 largest tubercle of the interradium. The ridges of the girdle in 

 Cidaris are thus composed of an upward growth and thickening 

 of the first and second interradial plates in one zone, and of the 

 first plate in the other zone, and there is no additional structure. 

 There are no structures in Cidaris which resemble or are homo- 

 logous to the processes of the girdle of tlie other families of the 

 regular Echinoidea. J. Miiller and C. Stewart have described 

 and drawn certain nodular projections, which are almost spines in 

 places, on the ambulacra! plates ; tlieir position is on either side 

 * Interradium 5 (the odd posterior) is chosen for illustration. 



