118 Prof. P. M. Duncan on the 



which cau?es the outer and inner pairs of pores of a triplet to 

 be so nearly on the same horizontal line. 



The middle series of pairs (c) confused or not in vertical 

 succession relates invariably to the upper or aboral plate of a 

 compound geometrical plate ; and this should not be forgotten 

 in investigating the distribution of plates in the very compli- 

 cated poriferous zones of the other species of the genus. 

 Each pair of the middle series is in a low but broad demi- 

 plate, and is placed remotely from the ambulacro-interradial 

 suture (fig. 14, c) . 



Every compound plate consists therefore of a low triple set 

 of pairs. It should be noticed that the pores are very oblique 

 in a peripodium. 



On examining the test from within, a different arrangement 

 of the pores is observed, but the great dissimilarity noticed in 

 Microcyi^hus does not occur. Nevertheless the pores of a pair 

 are much further apart than on the free surface, and the great 

 horizontal distance of the pairs is not recognized. 



Seen on the inside of the test (figs. 15, 16) the pairs of 

 pores are in zigzag, but the adoral pair can be recognized by 

 a certain vertical arrangement and by the inner or adoral pore 

 of the pair being large and at the end of a short groove ; the 

 other or aboral pore is small, circular, and is placed obliquely 

 upwards and outwards. Fig. 15 is a view, slightly diagram- 

 matic, of two compound plates near the apex, seen on the inside 

 of the test during the action of benzule. Tlie adoral pairs of 

 pores are readily distinguished, because the adoral pore is upon 

 the lower horizontal suture of the compound plate. The contact 

 of the adoral primary plate is seen with the ambulacro- 

 interradial suture. In fig. 16 the exclusion of the corre- 

 sponding plate a is seen, and in all plates a singular curva- 

 ture of the lower horizontal suture occurs, and it is curved up 

 towards the upper pore of the pair before passing outwards to 

 the ambulacro-interradial suture. This curving is very 

 anomalous, and is especially distinct when the primary is 

 excluded. 



The outer and more or less vertical row of pairs of pores, Avith 

 one of the pores close to the ambulacro-interradial suture, is the 

 middle pair of the compound, and is in a small demi-plate ; 

 its pores are much more horizontal when seen from within 

 than in the peripodium (figs. 15, 16, h). 'Jlie highest pair 

 of pores of a plate is further from the ambulacro-inter- 

 radial suture than the pair just described, and its pores are 

 variably placed as regards verticality, and they often overlap 

 the outer pore of the adoral pair and the inner pore of the 



