52 
PROF. P. M. DUNCAN AND MR. W. P. SLADEN ON TITE 
the compound plate at tlie median suture of the ambulacrum 
(fig. 4). (B.M. 180.) (3) There are two pig. 4 . 
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 median 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 Triplechinidse*. 
In nearly all the specimens the size of the pores diminishes 
actinally, hut 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 hiserial in their arrangement, 
and the pores are placed so obliquely that the aboral one is 
nearly vertical to the adoral. Although 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 ahactinal one of the series, consists of 
a large middle primary and a small aboral 
demi-plate and a larger adoral demi- ^ 
plate. The aboral demi-plate is perfo- 
rated by a pair of the outer set of pores 
of the hiserial zone, and the primary 2 
by a pair of the inner set, whilst the 
adoral demi exhibits a pair of pores 3 
similar to those of the aboral demi- 
plate. But the primary occupies the 4 
whole of the median suture, and the 
demi-plates do not come in contact ; for the adoral suture of the 
* Loren, ‘Etudes,’ pi. xvii. 
