THE BIOLOGY OF THE FUNAFUTI ATOLL AND REEF FORMATION. 135 



(2) Halimeda. This calcareous alga is abundant to an extraordinarily large extent 

 in the lagoon and on the ocean slopes. In order to determine its bathymetrical 

 distribution, systematic dredgings were carried on off the islets of Funamauu and 

 Falefatu, with the result that it was invariably found to be present alive down to the 

 45-fathom limit. Beyond that depth it was never met with alive, except in the case 

 of a specimen brought up in the tangles on one occasion off Tutanga islet from 

 80 fathoms. Of the numerous dredgings, however, which were made beyond the 

 45-fathom limit, this was the only instance of its occurrence alive below that depth. 



(3) The Corals and hydrocorallines. All stony corals can be said to be reef-forming 

 organisms. Most of them, however, are passive rather than active reef formers, 

 merely furnishing, when dead, material to the active builders. They are accessory 

 rather than essential ; the reef would exist without them, as in the case of the 

 Lithothamnion reefs of the Gilbert Islands. Others, however, do more than this ; 

 they are not only present in large quantity, thus forming a corresponding percentage 

 of the material out of which the reef is built up, but they actually lay the foundation 

 for further additions to the reef. Their skeletons are the framework, the spaces 

 between which are to be filled in by the reef builders. 



In order of the frequency ot the occurrence of these reef formers we have : 

 (a) Heliopora cceridea ; (6) the Millepores ; (c) the Porites family ; (d) Madrepora ; 

 (e) Pocillopora. 



(a) Heliopora cce7'ulea. This coral is at the present time, as large areas of its 

 dead remains indicate it to have been in the past, the most widely distributed coral 

 in the lagoon. On the entire rim of the atoll, however, it is of exceedingly rare 

 occurrence in a living state, a small stunted piece being met with only here and there, 

 On the lagoon slope, however, it is more frequently found, and towards the north- 

 eastern end of the atoll it begins to be very common, and in the neighbourhood of 

 the islet of Fualifeke there must be some acres of it alive. From this point westwards, 

 and thence in a southerly direction along the entire western rim, living stocks of it 

 are as frequent as they are rare on the eastern rim. 



Heliopora ccerulea was not met with alive on any of the ocean platforms except 

 at Amatuku, where it was specially sheltered by a small islet lying a little seawards 

 of that islet ^; nor was it found living by us on the submarine slope of either side of 

 the atoll, though the frequency of recent fragments on the ocean beach of Funamanu 

 probably imply that there is a patch of living Heliopora ccerulea in that locality. 



Mr. Gardiner* has mentioned that a single small piece of Heliopora was dredged 

 alive from a depth of 35 fathoms on the submarine slope off' the southern reefs of 

 the atoll. Water-worn fragments of a fresh and recent appearance, as far as the 

 colouration of the skeleton was concerned, were occasionally to be seen on the 

 Hurricane Bank in the neighbourhood of the 1897 bore. The fact, however, that not 

 a single specimen was obtained from the submarine slope of the atoll by the 

 ♦ 'Proe. Camb. Phil. Soe.,' vol. 9, 1895-1898, pp. 423 and 436. 



