ECOLOGY OF THE MURRAY ISLAND CORAL REEF. 41 



the branching species of Pontes can long survive being exposed to the air in 

 the hot sun, provided the basal parts of the colony remain immersed in sea- 

 water or in water-soaked mud. This, however, is of little service to the 

 compact corals and thus it is that, even though their basal parts be immersed 

 in salt water, an hour's exposure to the air and sun is sufficient to kill such 

 species as Seriatopora hystrix and Pocillopora bulbosa. 



The results of experiments upon this subject are shown in table 12. 



The Great Barrier Reef corals which survive exposure to sun and air 

 at low tides are of the genera Acropora, Montipora, Goniastrea, Symphyllia, 

 Cceloria, Mcsandra, Porites and other cavernated forms; and this fact will be 

 apparent to any one who inspects the numerous photographs of corals taken 

 by Saville-Kent showing the reefs laid bare at low tide. 



THE SOLUBILITY OF LIMESTONE IN SEA-WATER. 



The oolitic limestones of Florida and the Bahamas appear to be largely 

 derived from chemically precipitated calcium carbonate which has been 

 thrown down from the sea-water through the agency of bacteria in the man- 

 ner determined by G. Harold Drew,^ as amplified by the later studies of 

 Karl F. Kellerman and N. R. Smith, whose researches show that calcium 

 carbonate is precipitated in sea-water when a denitrifying or ammonia- 

 producing bacillus acts in conjunction with a form which produces carbon 

 dioxide. (See Journal Washington Academy of Science, vol. 4, p. 400, 

 August 1914.) 



In Torres Straits and the Murray Islands, on the contrary, the bulk of 

 modern reef-forming material consists of corals, shells of mollusca, foramin- 

 ifera, tests of echinoderms, the calcareous parts of nullipore algae and remains 

 of other organisms; the material derived from precipitated calcium carbon- 

 ate is unimportant, and oolitic limestones were not seen by me. 



The results of current-scouring and disintegration are seen in the region 

 of Torres Straits, in the many corroded, cavernated, dead coral stocks, and 

 above all in the disappearance of the lithothamnion ridge of fringing reefs in 

 all regions excepting the seaward edge where the breakers dash in full force. 



As the fringing reefs grow seaward the older (shoreward) parts of the 

 lithothamnion ridge corrode and disappear and thus the ridge remains merely 

 as a narrow crest along the advancing seaward edge of the reef-flat. 



The work of Mr. R. B. Dole- at Tortugas upon water samples collected 

 by Vaughan appeared to show that in this Florida lagoon the carbon dioxide 

 in the sea-water is either combined or half combined, none of it being free. 

 Later, Dr. Shiro Tashiro' casts doubt upon Dole's opinion that there is no 

 free CO2 in sea-water, but he did not disprove it, admitting indeed that if 



'Mr. Drew's latest studies are published in Papers from the Tortugas Laboratory of the Carnegie Institu- 

 tion of Washington, vol. 5, pp. 9-4S, Carnegie Inst. Wash. Pub. No. 182, 1914. 

 'Carnegie Inst. Wash. Pub. No. 182, pp. 69-78. 

 •Tashiro, Shiro, 1914, Carnegie Inst. Wash. Year Book No. 13, p. 220; Ibid., No. 14, 1915, p. 217. 



