326 J. STANLEY GARDINER. 



true reef flat or in the fissure zone of any Maldivan reef, freely exposed to the sea. They 

 are not important — if at all — builders on the outer slope down to 15 fathoms either against 

 the open ocean or the Great Maldive Sea. On the other hand they are found in great 

 clumps, up to 2 or 3 yards across, in boat channels and on sand flats as at Rotunia, usually 

 the colonies having more or less lamellate facies. The recorded dredgings, where the two 

 genera were obtained, were all in passages or outside atolls, and both were generally found 

 living together. Heliopora had previously been obtained by Basset-Smith off the Macclesfield 

 bank between 25 and 35 fathoms' and by the writer off Funafuti between 35 and 45 fathoms^ 

 and Millepora at about 25 fathoms, likewise off Funafuti-. Otherwise the genera were formerly 

 regarded as essentially reef or surface forms. It is interesting to observe that absolutely no 

 specimens of either were procured in the Maldives from between the surface and 20 ftithoms, 

 although Millepora at any rate was very abundant in all the passages into Funafuti lagoon, 

 to 10 fathoms in depth. The specimens of both forms were mostly in the form of wide plate-like 

 branches, although one incrusting specimen of Millepora was also secured. The quantity usually 

 obtained, when either of these forms was dredged, shows that they are not improbably the most 

 or among the most important of the reef-builders below a depth of 25 fathoms. 



Among the other builders requiring consideration the most important is Polytrema, the 

 significance of which lies in its power of binding together sand and rock. In the Maldives 

 growths were obtained in practically every dredging, in which corals or fragments of rock 

 were secured. In the passages Polytrema forms concentric incrustations round small pieces 

 of coral or rock. Outside the atolls in soundings and dredgings it was obtained down to 

 125 fathoms, and its importance is probably very great. Lithothamiiion was constantly j^resent 

 in dredgings outside reefs or in passages down to 45 fathoms, protecting dead coral or rock 

 below 25 fathoms. Indeed the dredgings show that it is chiefly in this last quality that 

 both this form and Polytrema are important, neither below 25 fathoms providing any con- 

 siderable bulk of the material of which the reefs are built. Halimeda leaves, serving to fill 

 in the hollows and the interstices of the other organisms, were constantly procured living 

 down to 35 fathoms, beyond which its growth appeared to gradually die out. In Mahlos 

 great beds of 3'oung oysters (apparently Meleagrina margetifera) were found at 25 fathoms, 

 the shells of %vhich would be of assistance. 



Solitary corals have in the list been completely neglected, most genera having only 

 a minute value in the formation of reefs. Fungia is sometimes found in great abundance 

 on surface reefs, but was only once dredged, i.e. between 16 and 20 fathoms. An allied 

 genus, Diaseris (probably synonymous with Gycloseris), was obtained living on nine occasions 

 between 25 and 40 fathoms. In five of these dredgings the quantity of coralla — principally 

 dead but also living — was very large, and in a tenth haul only dead specimens were secured. 

 In the north passage into Suvadiva lagoon, 36 to 40 fathoms, in one haul of a 4^ feet 

 beam trawl about 2 cwt. of dead masses of rock were brought up, mainly composed of the 

 dead coralla of this genus, cemented together by sand and deposited carbonate of lime. 

 A second dredging of the same passage, where it opens into the lagoon, gave great quantities 

 of dead Heteropsammia in addition to a number of live specimens. In the same position 

 the Snapper lead three times in succession brought up specimens of the dead coralla of 

 this genus, the whole bottom appearing to be almost coated by them. 



' Ann. Mag. Nat. Hist., Nov. 1890, p. 353 et seq. - Loc. cit. pp. 478—9. 



