AMBULACRA OF THE RECEITT DIADEMATIDJ3. 109 



tlie great plate. The third plate of the triplet is large, and 

 intrudes much on the space which is occupied by a second plate 

 in compound plates placed higher up in the test. 



The next great plate, C, differs in detail from the others. 

 The first set of triplets (5) is without a tubercle, and the first 

 of the plates is a long and low primary, with the adoral suture 

 almost transversely placed so that a small portion of the median 

 suture is in contact with the plate. The second plate occupies 

 all the rest of the surface of the compound plate towards the 

 median line, for the third plate is a demi plate. The second set 

 of triplets (6) has a tubercle on it, and the direction of the 

 sutures and the shape of the plates are the same as those of the 

 first set of triplets of the great plate A. 



On studying the ambulacra from the outside, but little can be 

 learned regarding the shape of the plates of the triplets ; but it 

 is evident that the extremely close sutures noticed from within 

 are more separated externally. The great plate C was rendered 

 sufficiently permeable by light to trace the sutural lines, and some 

 interesting points became evident (PI. Y. fig. 9). 



Commencing with plate 1 of the aboral triplet, its shape corre- 

 sponds with that noticed from within ; the adoral suture crosses a 

 large secondary and a smaller tubercle to reach the median liae. 

 Plate 2 extends beyond plate 3, towards the interradium, and 

 its adoral suture curves at first abactinally, so as to touch the 

 boss of the larger of the secondary tubercles, and then actinally . 

 It then slopes to the aboral edge of the great tubercle of the 

 next set of triplets, conforms to it, and reaches the transverse 

 suture placed between the first and second set of triplete. 



The third plate of this first set of triplets does not appear to 

 belong to it, but to make up a plate of the second set. Never- 

 theless the plate is part of the first set, and the transverse 

 suture of the half great plate limits it adorally. It is a demi 

 plate, for it does not touch the median line. 



The structure of the outside of the plate is rendered difficult 

 of comprehension, because the great tubercle on the second set 

 of triplets has grown so as to overlap and cover much of the 

 third plate of the first set. 



Plate 1 of the second set of triplets is a long low primary with 

 its adoral suture crossing the aboral shoulder of the ridge at the 

 base of the mamelon, and reaching the median line just adorally 

 to the junction of the upper and lower triplets. 



