42 PROF. p. M. DUNCAN AND MR. W. P. SLADEN ON 



therefore demi-plates, wliilst tlie central plate is a primary of 

 great dimensions. 



This central plate has a low poriferous portion, but the rest of 

 it is greatly expanded, so that all the compound plate which is 

 internal to the adoral and aboral demi-plates (that is nearer the 

 median suture of the ambulacrum) belongs to it. The sutures of 

 the demi-plates do not unite in any way. Guided by the view 

 of the sutures of the compound plate as seen from within the 

 test, it is not difficult to trace their line on the outside, either 

 directly or by mapping out (fig. 8). The limitation of the 

 aboral demi-plate to the upper and outer shoulder of the tubercle 

 is very evident, and so is the position of the adoral demi-plate on 

 the lower and outer shoulder of the tubercle. The parts of the 

 plate and tubercle placed between the demi-plates, comprising 

 the slope of the tubercle up to the mamelon, the mamelon itself, 

 and all the rest of the tubercle towards the median line, belong 

 to the great central primary plate of the triplet. 



On referring again to the view of the test from within, it may 

 be observed that the marking of the sutures on the inner side of 

 the ambiilacra is so distinct that the shape of the compound plates 

 is readily drawn. The compound plate is, taking the largest as 

 the type, high and hexagonal in shape, the aboral and adoral sides 

 being the smallest. Four pairs of pores are in relation to the 

 compound plate, but only three belong to it. The inner or 

 adoral pore of each pair is at the end of a long groove, indicative 

 of the growth of the plate. The abora,lly placed pair belongs to 

 the tubercle-bearing plate above, and the transverse suture of these 

 successive plates crosses the adoral pore of the pair. The next 

 pair, which is the proper 1, of the triplet, is larger and is situated 

 nearer the interradium than the pair just noticed. Pair 2 is of the 

 same size as pair 1, and the pores are placed almost directly beneath 

 those of the first pair but at some distance. The pair 3 has 

 smaller pores, and they are placed on a vertical line beneath the 

 aborally placed pair, and therefore to a certain extent nearer the 

 median line than those of the pairs 1 aud 2 (PI. I. fig. 8). The 

 inner suture of the aboral demi-plate is seen to pass from the 

 adoral pore of pair no. 1, almost directly upwards, to reach the 

 adoral pore of the last pair of the triplet in relation to the com- 

 pound plate immediately above. 



The corresponding inner suture of the adoral demi-plate passes 

 from the adoral pore of the second pair towards the adoral pore of 



