THE BIOLOGY OF THE FUNAFUTI ATOLL AND REEF FORMATION. 133 



At Pava the submariue slope was investigated adjacent to, as well as facing, tlie islet. 

 It differed from those already described in the presence of a greater quantity of corals 

 and hydrocorallines, and this difference was most marked near the surface, where the 

 massive ridge-like Millepora complanata, together with various other corals, forms 

 shoals closely resembling in their mode of origin those in the lagoon. Some of these 

 shoals are from 1 to 2 fathoms in height ; they arise from the reef rock below and have 

 a vertical wall facing tlie atoll rim, while towards the ocean they descend in a gentle 

 slope. Between the ridges and shoals there is much Hcdimeda sand, together with 

 boulders, in shape and form like enormous smooth pebbles. In the deeper water 

 there was much living Hcdimeda and great patches of non-calcareous green seaweed. 



We have already referred to tlie very characteristic difference between the upper 

 portions of the western and eastern rims of the atoll, as regards the nature and condi- 

 tion of the reef- forming organisms. No reason whatever could be found on the spot to 

 account for this. Indeed, as regards wind and current, and therefore food supply, every- 

 thing seems much more favourable on the eastern rim for a more vigorous growth. 

 The fact that almost the entire rim of the eastern side is occupied by islets might 

 suggest a possible exclusion of food material from the lagoon, but its water on being 

 tested did not show the increase in carbon dioxide which would have been present 

 had any staleness, due to inadequate communication with the ocean, been the cause of 

 the impediment in the growth of the organisms. My former teacher. Professor 

 Haswell, suggested that nothing short of an epidemic amongst the various organisms 

 could have worked such a change. 



Possibly there may be such a thing as a culminating point in the development of 

 an atoll — a point beyond which the growth of the reef- forming organisms may decline. 

 A period of constant water-level might possibly conduce to this, i.e., a general 

 degeneration of all reef-forming organisms. The appearance of the entire eastern rim 

 points decidedly to a retrogressive stage in the growth of these organisms.* 



(3) Distribution of the Chief Reef- Forming Organisms. 



Grouped according to their importance we have : (1) Lithotham7iion ; (2) Halimeda ; 

 (3) the Foraminifera ; (4) the Corals. 



(1) Lithothamnioji.'t There are three distinct species at Funafuti : (a) The 



* Geological evidence shows that recently there has been heavy silting in this part of the lagoon on the 

 eastern side of the atoll, strip after strip of beach sand and gravel being added to the lagoon shore, as 

 shown on Plate 9. Perhaps the death of Heliopora avrulea in this part of the lagoon is due to this 

 silting.— T. W. E. D. 



t It is doubtful whether a sufficiently important place has been assigned here to the Foraminifera : and 

 possibly Halimeda should take precedence of Lithothamnian, if the basis of classification be entirely 

 quantitative. In view, however, of the all-important part played by the Lifhotliamnion in consolidating the 

 loose material of the reef, and so building reef -rock, it has a strong claim to the priority assigned it. — 

 T. W. E. D. 



