52 PROF. p. M. DUJfCAN A^D MR. W. P. SLADEIT OX THE 



tlie compound plate at tlie median suture of the ambulacrum 



Fig. 4. 



(fig. 4). (B.M. ISO.) (3) There are two 

 demi-plates ; the upper one has its aboral 

 suture passing actinally so as to reach the 

 adoral suture of the compound plate at a 

 little distance from the mediaii suture ; 

 whilst the lower demi-plate has its aboral 

 suture also sloping actinally and reaching 

 the common adoral suture not far from the interradial edge. 



The position of these demi-plates is exactly opposite to that 

 noticed in the compound plates of the Triplechinidae*. 



In nearly all the specimens the size of the pores diminishes 

 actinally, but this is not invariable; and in all cases the pairs 

 become closer, and alternate pairs are crowded out of the straight 

 vertical line. There is no intercalation of pairs — that is, no new 

 growths amongst the original pairs. Actinally, and on the slope 

 of the raised ambulacra, which have already been noticed, the 

 pairs of pores are crowded and biserial in their arrangement, 

 and the pores are placed ?o obliquely that the aboral one is 

 nearly vertical to the adoral. jMthough the crowding of the 

 pairs is often excessive, still there is never more than a pair to 

 a plate ; and the plate, often very small, forms part of a com- 

 pound plate. Solitary primary or demi-plates which do not enter 

 into the composition of compound plates are not found below the 

 ambitus. The compound plates there consist of a primary with 

 one or two demi-plates. 



The following is a description of four compound plates placed 

 near the ambitus and actinally (fig. 5). (B.M. 38742.) Plate 1, 

 the abactinal one of the series, consists of 

 a large middle primary and a small aboral 

 demi-jDlate and a larger adoral demi- 

 piate. The aboral demi-plate is perfo- 

 rated by a pair of the outer set of pores 

 of the biserial zone, and the piimary 

 by a pair of the inner set, whilst the 

 adoral demi exhibits a pair of pores 

 similar to those of the aboral demi- 

 plate. But the primary occupies the 

 whole of the median suture, and the 

 demi-plates do not come in contact ; for the adoral suture of the 



Fig. 5. 



*Loven, 'Etudes,' pi. xvii. 



