THE BIOr.OGY OF THE FUNAFUTI ATOLL AND REEF FORMATION. 129 



filameutous form and browuisli -green in colour. This occurs along the entire length 

 of the platform, parallel to the shore, as a complete strip, like a slippery carpet. This 

 area, indicated as the seaweed zone, is left uncovered by water at low tide several 

 days before and after spring tides. No coral growth is, therefore, found on it. 



Between this zone and the edge of the platform lies the Lithothamnion zone (see 

 Plate 17). The line of junction between the two is very irregular, depending entirely 

 on the distance to which the waves wash landwards at low- water spring tides. This 

 distance is the limit of Lithothamnion growth, and varies with the nature of the 

 surface. 



The Lithothamnion zone thus formed comprises that part of the platform which is 

 so characteristically interrupted by the channels extending landwards from the edge 

 of the reef While the surface of the seaweed zone is comparatively smooth and level, 

 that of the Lithotha-mnion zone is exceedingly uneven on account of the slight mound- 

 like elevations formed partly of LMhothamnion and partly of encrusting foraminifera 

 like Carpenteria and Polytrema which almost invariably surround the channels, 

 especially at their terminations. These, like the entire surface, are very much pitted 

 with irregular shallow hollows, which, towards the edge, are inhabited by echinoids. 

 The whole presents an irregularly pitted surface, but with a total absence of sharp 

 edges, every jDart being smoothed over and rounded off by the growth of the Litho- 

 thamnion. The only species represented, however, is the lichenous form ; the other 

 two are totally absent.* As is the case on the lagoon jjlatform, it has not the healthy 

 looking crimson colour of this Variety of Lithothamnion when growing on the western 

 rim of the atoll, and it altogether presents a stunted appearance. Of coral growth 

 there is exceedingly little on the surface of the platform ; only here and there a 

 small corallum of Pocillopora grandis, hardly ever more than a few inches in diameter, 

 and, in most cases, coated to a greater or less extent with Litiiothamnion. Pocillopoi'a 

 verrucosa and clavaria are also present, but still less frequently. In the channels, 

 which were in no instance more than 13 feet deep, and therefore easily examined, 

 there was a greater variety of coral, but nowhere in anything like large masses. The 

 walls of most of them showed YLothvughut Lithothamnion growth ; the bottom consisted 

 of large pebbles and coarse sand. This was altogether contrary to expectation in view 

 of the fact that this platform is on the windward side of the atoll and fully exposed to 

 the prevailing ocean current. The chief reef- forming corals, Porites limosa, Heliopora 

 ccerulea and the hydrocoralline Millepora alcicornis of other localities in this atoll 

 were not once met with alive on the ocean platform in this part of the atoll. The 

 note on Plate 17 as to the occurrence of Millepora alcicornis on the line of section 

 A to B, across the channels, relates to a dead specimen of this species. 



The evidence for the description of the platform and channels was obtained by 



* It is clear, however, from the section afforded by the diamond-drill bore, as well as from large masses 

 of dead reef rock lying on the reef platform north of the main diamond-drill bore, that a branching type 

 of Lithothamnion flourished not long ago at this part of the atoll. — T. W. E. D. 



S 



