THE FAMILY AEBACIADiE. 29 



test, they are simple and primary when first produced, and that 

 as there is a gradual growth of plates from the radial-plate end 

 towards the peristome during the whole life of the individual, 

 the increasing pressure from above downwards, or aborally 

 adorally, produces absorption of, or interferes with the growth 

 of, the inner parts of some plates and develops the demi- 

 plate. 



The demi-plate intercalated between the large primaries in a 

 compound or multiple plate, is not an independent or super- 

 added structure, but a modification of a primary plate during the 

 growth of the ambulacrum as a whole. 



Loven has shown, in his ' Etudes,' pp. 23, 24, that even the 

 apparently confused doubling and trebling of the arcs of the 

 pairs of pores near the peristome in some adult Echini, is not 

 due to the development of new pore-bearing plates, but to a 

 downward movement and diminution of the height and breadth 

 of the original plates accompanied by a movement of the pairs of 

 pores, especially of those in the larger plates of the compound 

 plates. There is no real confusion, for the pairs of pores assume 

 their places in a regular order. 



Coelopleurus. Megion of the Ambitus. — If a large tubercle- 

 bearing plate of an ambulacrum of one of the species of Ccelo- 

 pleurus already named be carefully examined, there is no difficulty 

 in deciding that it is really a multiple or compound plate, and 

 that the arrangement of the three plates which compose it is 

 totally different from that as yet observed in any other genus. 

 Before describing the three plates, it is necessary to remark that 

 the tubercle has a large flat edge to its base, which comes so close 

 to the poriferous zone that the inner pores of the pairs of the 

 triplet intrude on it, and as it were slightly erode it. The 

 tubercle is large, covers nearly all the rest of the plate, and slopes 

 up to a circular groove which is at the narrow base of a mame- 

 lon which is very small in relation to the size of the boss beneath. 

 In some instances the base of the tubercle is not thus intruded 

 upon (PI. I. fig. eS). 



On examining the three plates of this compound and tubercle- 

 bearing plate, it is found that there is only one primary {a), and 

 on the other hand there are two demi-plates {V , h"). Moreover 

 the upper or aboral plate and the lower or adoral plate of the 

 compound plate are demi-plates, the large single primary being 

 between them and extending far beyond them towards the inner 

 edge of the plate at the median line. 



