ORGANISMS OF CORAL REEFS 395 



AT. coiiiplaiiafa. The first is restricted to the lagoon side, the second 

 also occurs on the ocean side. Among the corals Hcliopora cocrulca 

 is by far the most important, but is scarcely found on the outer rim 

 of the atoll. It occurs on the lagoon side, where it often forms the 

 chief lime contributor next to the alg?e. The yellow Poritcs liinosa 

 and a purple species of Forites also occur on the lagoon side, but 

 seem to be rare or absent on die ocean side. Large blocks of rock 

 made of dead Forites abound in some sections, showing this coral 

 to have been formerly of great importance. Besides these species, 

 the branching Madrcpora loriccps abounds on the western rim of 

 the atoll, but is rare elsewhere. Focillopora of several species also 

 occurs. Astrsan corals seem of far less importance at Funafuti 

 than at other coral reefs. The reef platform has a width of 1,300 

 yards where it supports the islet of Fualopa, but it is mostly much 

 narrower. It consists of a smooth, barren surface formed on dead 

 Lithothamnion coral sand, and coral fragments, generally overlying 

 a basal stratum of dead Helipora cavulea, Forites or both. In 

 many cases this old Helipora reef still forms the surface of the 

 platform within the Lithothamnion fringe, the islet being built upon 

 it by the accumulation of coral and Lithothamnion sand and breccia. 

 A dense growth of sea weed is characteristic of the submerged por- 

 tion of the platform. 



In general, the ocean rim of the leeward portion of the atoll 

 is characterized by the enormous quantity of branching Litho- 

 thamnion with its vivid coloration, the great abundance of Madre- 

 pora loriccps, the general small size of all the species and the ab- 

 sence of Hcliopora cocrulea and Millcpora alcicornis. The plat- 

 form is smooth and not divided into zones, while the transverse 

 channels are short and shallow. 



On the windward side, the branching and knobby forms of 

 Lithothamnion are wholly absent, while the lichenous form present 

 has a dark color. Corals are scarce, Madrcpora loriccps, Hcliopora 

 ccerulea and Millcpora alcicornis being wholly wanting, while Focil- 

 lopora grandis, though rare, is the predominant type. No large 

 masses of coral are, however, found. The platform is divided into 

 erosion, corrosion, seaweed and Lithothamnion zones, while trans- 

 verse channels are numerous, deep and long. 



From the platform surface rise the islands built by the calcare- 

 ous debris derived from the destruction of the corals and nulli- 

 pores and the old reef rock. As might be expected, the islets are 

 most abundant on the windward side of the atoll, where by far the 

 greatest part of the rim is covered by them, while on the leeward 

 side they are few and much smaller. The leeward islets consist of 



