AMBULACRA OE THE RECENT DIADEMATID^. 97 



iu there and low. Thence the sutures gradually diverge and reach 

 the median or vertical suture of the compound plate at points 

 very distant one from the other. One suture, that between the 

 first and second plates of the triplet, reaches the inner or median 

 edge of the compound plate, close to and below its abactinal 

 angle (Plate V. figs. 1 and 2). The other suture, or that be- 

 tween the second and third plates, attains the median edge close 

 to and above the actinal angle of the compound plate. 



As both of these sutures reach the median line, all the plates 

 of the triplet are primaries, and there is no demi plate. 



In consequence of the divergence of the sutures internally to the 

 mamelon of the compound plate, the second, or central plate, is 

 the largest, and carries the greater part of the tubercle ; the other 

 two plates are pushed adorally and aborally and have sufi'ered 

 compression from above downwards during growth, and they are 

 low plates, except where they expand in height at the region of 

 the tubercle. (Fig. 3.) 



The triple combination therefore consists of a long low pri- 

 mary, of a long and internally expanded and tall middle primary, 

 and of a third plate resembling the first plate, more or less. 



In the figure (Plate Y. fig. 1), it will be observed that the line 

 of suture between the first and second plates does not pass from 

 the adoral pore of the first pair ; this is owing to the growth of 

 the plates in height. The corresponding defect in the instance 

 of the adoral pore of the other pairs is due to the same cause ; 

 but originally the lines of suture were, as is usual in all Echini, 

 and as is indicated on fig. 2. 



Throughout the ambulacra the compound plates present varia- 

 tion in their height and breadth, but the succession of the three 

 primaries of the triplets is always the same. The second plate 

 is never blocked out from the median line by a union of the 

 sutures between the first and the third plates. 



Usually the sutures cannot be traced on the outside of the 

 compound plate, and the sides of the tubercles are free from any 

 markings ; but the application of benzole will sometimes indicate 

 the lines of junction of the plates, and then the mamelon will be 

 seen to lie between the sutures and to belong to the second plate. 

 (Plate V. fig. 3.) 



The distinction between the arrangement of the triplets in 

 Diadema and the Arbaciadse is evident, for the first and third 

 plates of the compound plates of the ambulacra of Diadema 



