DEVELOPMENT OF THE REEF 243 



is certainly the greatest enemy to activity, and the most 

 potent cause of death in corals. 



It may therefore be said to be an axiom in coral bio- 

 nomics that an area in which sedimentation is taking 'place is one 

 in luhich the ree^-huilders ivill not flourish. It is then possible 

 to say that reef- building corals will not live below the 

 Limiting Line of Sedimentation, and this without special regard 

 to the nearness of reefs, or the presence of land-cutting 

 currents, but as a general statement. I have said that we do 

 not know precisely where this line may be, and that it will 

 vary in different places from purely local causes, but wherever 

 it may be reef-coral growth will be excluded from the depths 

 below it, and the surface water above it will — other condi- 

 tions being favourable — oifer a suitable site for their develop- 

 ment. We may therefore lay it down as a law that the 

 hathymetrical limit of the reef-huilding corals extends to the limiting 

 line of sedimentation, and that this exact level will be the 

 outcome of purely local conditions. 



It may be objected that, if the reef-building corals are pre- 

 sumed to be unable to live below the limiting line of sedimen- 

 tation, how is it that many species of solitary corals are found 

 in far deeper water ; for if sediment is the real cause of death, 

 how do some species resist its action ? 



There seems to be but little doubt that it is the common 

 type of growth of these deep-water corals which explains their 

 ability to withstand the action of silt. Most of them are flat 

 discs, or more or less wide-mouthed cups : they are not, as a 

 rule, honeycombed with a series of small openings, as are the 

 reef-builders. It would seem at first sight that this type of 

 growth was perhaps the most ill adapted to resist the effects of 

 sedimentation, for a larger proportion of the zooid surface is 

 exposed to the dropping particles. The large living specimens 

 of the Fungice, which lie scattered about in sheltered pools on 

 the barrier flats, afford the best examples of the extreme degree 

 of flattening of the corallite, and exposure of the zooid : the 

 study of the habits of these Fungice offers the explanation of 

 their ability to live in a silting environment, 



