446 PEOF. p. M. DUXCA2?- OX THE STErCTTTRE OF 



The aboral pair, «, like tlie others, is oblique and in a peripodinm. 

 It belongs to the inner series, and is separated from the interradial 

 suture by a rather large space, which is ornamented. The adoral 

 pore of the pair is on the line of suture between the poriferous plate 

 and the next in adoral succession, and the suture is curved with the 

 convexity adoral, so as to reach the flank of the boss and thence 

 to pass inwards and aborally, to the median line, and close to the 

 aboral angle of the geometrical compound plate. This plate, with 

 the aboral pair of pores, is of the Diade-ma-sha-pe : but the pores are 

 remoter from the interradium than is the case in the genus Pseudo- 

 diadema. 



The next pair of pores, 6, placed actinally, is close to the inter- 

 radium, belongs to the outer set, and is in a peripodium. The plate 

 is nipped in just adorally to the position of the first pair, and it then 

 expands so as to include the mamelon and much of the boss and form 

 the greater part of the compound plate near the median line. 



The adoral pore of the pair is in contact with the suture at the 

 edge of the plate below, which is curved with the convexity aboral, 

 and the suture reaches the median line slightly aborally to the 

 actinal angle of the compound plate. This suture is the limit of 

 the third plate, c, of the combination, and this has its pair of pores 

 forming part of the inner series. The adoral pore of the plate (c) is 

 on the transverse suture placed actinally to the compound plate. 

 Finally the actinally placed plate, d, is a demi-plate which may seem 

 to have nothing to do with the combination ; but it forms a small 

 portion of it, and the pair of pores belongs to the outer series. 

 The plat« does not reach further towards the median line than the 

 position of the adoral pore of the pair of the third primary' plate. 

 There is no doubt that such a combination is not found in the 

 genera Diadema and Pseudodiadema. 



Nearer the ambitus, or at that spot, the usual number of pairs of 

 pores to a compound plate is four, the line of the pairs is simple, and 

 the distribution of the composing plates is as in the species of 

 Remicidaris, that is, there are four primaries, of which the third 

 is the largest (fig. 20). 



Fig. 20 (see p. 452). 



In some plates, however, there is a; demi-plate ; and then 

 the structure rather recalls the compound ]3latesJof the Plesio- 

 diademata. 



The doubling of the pairs of pores towards the peristome is 

 almost a copy of that seen near the radial plate ; there is no addition 



