CORALS AND CORAL REEFS — VAUGHAN. 197 



rence. As there are, particularly along the sides of channels through 

 which water flows into and out of lagoons, situations intermediate in 

 condition between those in the lagoons and those on the exposed sea- 

 sides of reefs, there are areas in which there is more or less com- 

 mingling of the two kinds of corals, and in them both massive reef- 

 building forms and fragile lagoon forms live side by side. 

 RELATION OF CORALS TO DEPTH OF WATER. 



A great deal of information has been accumulated on the relation 

 of corals to depth of water. Among those who have particularly 

 studied this subject are Darwin, Dana, Pourtales, Quelch, Moseley, 

 Stanley Gardiner, and myself. Usually massive reef builders are 

 mostly found in water 27 meters or less in depth, but some species 

 extend to depths between 37 and 48 meters, and a few reach depths as 

 great as 74 meters. The available evidence indicates a depth between 

 37 and 46 meters as the maximum at which a true coral reef will form. 



At depths slightly greater than 46 meters, between 46 and 74 

 meters, there are in coral-reef areas corals that differ somewhat from 

 the shoal-water fauna and from the true deep-sea corals. These 

 corals naturally resemble more closely those found in the deep water 

 of the lagoons than those on the exposed sides of reefs or the flats 

 just behind exposed reefs. Stanley Gardiner appears to have been 

 the first clearly to recognize this bathymetric faunal zone, and in 

 his work on the Maldive and Laccadive Archipelagoes very properly 

 emphasized its importance. In my own work on the living corals 

 of the Hawaiian Islands, I recognized the presence of a rather dis- 

 tinctive fauna at these depths. Illustrations of it are given on 

 plates 9-11. 



Between 74 and 183 meters in depth corals of deep-sea f acies com- 

 mingle in the Hawaiian Islands with the fauna found principally 

 between 46 and 74 meters in depth. Deep-sea corals, those found 

 in water 183 meters or more in depth, are mostly simple, cup 

 corals, and many have very delicate, fragile, even lacelike skeletons. 

 Several species from the Hawaiian Islands are illustrated by plates 

 12-15. A species that closely resembles the one illustrated by 

 plate 14, figures 3, 3a, was dredged off Callao, Peru, in water 3,209 

 fathoms (=19,254 feet=5,892 meters) deep. Other deep-sea corals 

 are compound forms that have delicate, elongate, attenuate branches. 

 Three species with this kind of growth habit are illustrated by 

 plate 15. 



The following tables present the results of a study of the distribu- 

 tion of Hawaiian corals according to depth. Similar relations pre- 

 vail in the Indian Ocean, the Central Pacific, and in the Gulf of Mex- 

 ico and the Caribbean Sea. Although these tables apply specifically 

 to the Hawaiian Islands, they really illustrate certain of the broad 

 principles underlying the relation of coral faunas to depth of water. 

 65133°— sm 1917 14 



