THE BIOLOGY OF THE FUNAFUTI ATOLL AND REEF FORMATION. 131 



MiUepora alcicornis are absent. (5) The even and almost level surface of the entire 

 platform, together with the absence of a division into zones. (6) The comparative 

 scarcity, shortness and shallowness of the " cliannels " in the reef platform. 



Those of the latter are : — (1) The total absence of the branching and knobby forms 

 of Lithothammon. (2) The peculiar darkish colour of the Lithothamnion present, 

 which is the lichenous form. (3) The scarcity of coral, together with the total 

 absence of Madrepora loripes, Heliopora ccerulea, and MiUepora alcicornis. The 

 only corals typically present (although scarce) are Pocillopora grandis. (4) The total 

 absence of coral of any description in large masses, either singly or collectively. 

 (5) The definite division of the platform into erosion, corrosion, seaweed and 

 Lithothamnion zones. (6) The great number of " channels" and their extent. 



At the north and south ends of the atoll there is a slow gradation from the leeward 

 into the windward type of platform. 



Another great distinguishing feature between the two sides of the atoll is the fact 

 that, while on the windward side by far the greatest part of the rim is covered by 

 islets, these on the leeward side are few and are much smaller than any present on the 

 opposite rim. 



Where there are no islets the leeward rim passes gradually from an area covered 

 by living organisms nearest the ocean, into a central portion of perfectly bare and 

 smooth Lithothamnion rock. Thence to the edge next the lagoon the conditions of the 

 ocean face of the reef repeat themselves, but on a smaller scale. 



The windward rim, for instance, between the islands of Funafuti and Fatato, where 

 its nature is not obscured by the accumulation of debris to form land, is narrow and 

 ends both lagoonwards and oceanwards in peculiar buttress-like tiers. This seems 

 clearly to indicate its origin from clusters of coral which have been transformed into 

 Lithothamnion rock. There are no areas of living coral at its edges, but small pieces 

 of Pocillopora are fairly frequent ; also lagoonwards, clusters of MiUepora alcicornis 

 in the deeper water. There is no lagoon platform to the eastern rim, except opposite 

 to the islets. 



To the north of the islet of Mulitefala the rim is of great width. Oceanwards the 

 platform is intermediate between the typical windward and leeward forms. Lagoon- 

 wards it has the features characteristic of lagoon platforms which are being added to 

 by the growth of Lithothamnion upon the coral clusters. 



There are no true channels into the lagoon in the windward rim ; two ship passages 

 are present, but these are due to considerable stretches of the rim between the islets 

 dipping below low- water mark. The passages in the leeward rim, however, are true 

 passages, being clear cuttings, very narrow and with walls vertical in places, through 

 the rim. On these vertical walls large masses of MiUepora alcicornis are seen in a 

 flourishing condition, and the fact that this hydrocoralline is one of the frailest, seems 

 to indicate that the current flowing through the channels has no damaging influence 

 on coral growth. 



S 2 



