32 PROP. p. M. DUNCATST AND MR. W. P. SLA^BEK ON 



occupies the inner half of the tubercle and of the compound plate, 

 and comes to the inner or median suture of the triple plate. 

 This part is limited aborally and adorally by the transverse 

 sutures common to its plate and the compound plates which are 

 placed immediately above and below. 



Tie Amhulacra near tie Peristome.— The tubercle-bearing or 

 compound triple plates become smaller, lower, and narrower 

 towards the peristome, and quite at the margin there is some 

 turning in of the tirst plate in each zone, and a broad concavity 

 at the ambulacral median line. On either side of the ambulacra 

 is a small branchial cut, and a tall tag which is broad at the 

 margin and long vertically ; the latter is partly on an interradiura 

 and partly on an ambulacrum, and the cut is a shallow groove 

 without ornamentation. 



As the shape of the triple compound plates is lower near the 

 peristome, the two demi-plates and the intermediate primary 

 occupy less vertical space ; the pairs of pores are also closer 

 together (PI. I. fig. 7). 



The diminution of the breadth of the triple plate is accompa- 

 nied by an inward passage, that is towards the median line, of the 

 pairs of pores and also of the now small tubercles. The tubercles 

 close in, as it were, towards the median line, and the closing in 

 of the pairs of pores upon them is accompanied by an alteration 

 in the shape of the demi-plates. They become nearly rectangular 

 in shape, and the angles of their inner sutures are right angles ; 

 the direction of these sutures is vertical, and those which are 

 in contact with the poriferous part of the central primary are 

 nearly transverse, and not on the slant as they are higher up. 



It was noticed that the pairs of pores are oblique on the edge 

 of the large tubercles ; now this obliquity increases in the pairs 

 as they approach the peristome, and the adoral pores have the 

 aboral nearly vertically above them, there being a slight trend 

 externally (PI. I. fig. 10). The amount of obliquity varies, but 

 it is always decided. Close to the margin, the original first pair 

 of pores usually has its adoral pore as a mere indentation, and it 

 may happen that the third pair is on an oblique line with the 

 second and somewhat pressed out of the order; but there is 

 never any formation of secondary arcs of pairs of pores. The 

 marginal pores are very small and close. The pairs are remote 

 from the interradial edges of their plates which are on the area 

 of the tag;. 



