OF WESTERN SIND. 291 



bounded above and below by the sutures of the demi-plates, and further, towards the 

 median line of the ambulacrum, it forms the whole of the rest of the tubercle and the 

 granular edges. 



The size of the poriferous part varies according to the position of the edge of the 

 tubercle, and frequently the adoral pore comes in contact with and even indents it. Then 

 the size of the poriferous part is small. The other part occupies the inner half of the 

 compound plate, and therefore the greater part of the tubercle, all the mamelon, and 

 the greater part of the groove around it. A line drawn vertically, over the top of the 

 mamelon and reaching from the aboral to the adoral suture of one of these tubercle- 

 bearing plates, will nearly bound the inner expansion of the central or primary plate, 

 all between the line and the inner sutures of the plate belonging to it. So that the 

 large central primary plate of the triple combination, or compound plate, consists of a 

 narrow outer part placed between the demi-plates, and of a large inner expansion which 

 extends over the inner half and more of the great tubercle. 



Near the peristome, where the height of the triple combinations diminishes, the 

 aboral and the adoral demi-plates are more rectangular in shape, and the inner line of 

 suture of both of them is nearly vertical in its direction. In this region the central or 

 primary plate of each combination is small, although it bears the bulk of the boss and 

 all the mamelon. Whilst in all parts, of the ambulacrum remote from the peristome, 

 the plates are perforated by the aboral pore of the pair close to their outer or interradial 

 edge, near the peristome the perforation is remote and nearer the median line, the outer 

 edge of the plate forming a part of the " tag." 



There is some slight obliquity of the direction of the pores of each pair in relation 

 to the first large tubercle-bearing plate, and the aboral pores are at a slightly higher 

 (abactinal) level than the adoral. This obliquity is more distinct at the second tubercle, 

 and its lower pair of pores are often decidedly on a slant, the aboral pore being higher 

 up the test than the adoral. 



The obliquity increases in every pair of pores towards the peristome, and close to 

 the margin the aboral pore is nearly vertical to the other, but a little on one side of the 

 vertical line. 



There is some variation in the amount of the obliquity of the pores near the 

 peristome, but it is always decided. Moreover there is an interesting alteration in 

 the relative position of these peristomial pores, for instead of being close to the 

 base of the tubercle, as in the large tubercles, they are decidedly removed from it, 

 and the adoral pore does not touch the tubercle. 



Again, the aboral pores of the large tubercular plates are close to the edge of the 

 poriferous^ plate, but as the tubercles diminish in size towards the peristome, the aboral 

 pore gets further away from the interradial edge, so that there is some extent of plate 

 between it and the edge. 



This closing in of the pores upon a contracting tubercle is accompanied by an 

 alteration in the shape of the demi-plates of each triple combination. The demi-plate 

 above, and also that below the tubercle-bearing one, become nearly rectangular in 

 shape, the angle of the sutures (inner) is nearly or quite a right angle, the direction of 



2e2 



