CAUSES AFFECTING THE GROWTH OF CORALS. 119 



III. LOCAL CAUSES INFLUENCING DISTRIBUTION. 



Coral making species generally require pure ocean water, 

 and they especially abound in the broad inner channels among 

 the reefs, within the large lagoons, and in the shallow waters 

 outside of the breakers. It is therefore an assertion wide from 

 the fact that only small corals grow in the lagoons and chan- 

 nels, though true of lagoons and channels of small size, or of 

 such parts of the larger channels as immediately adjoin the 

 mouths of freshwater streams. 



There are undoubtedly species especially fitted for the open 

 ocean ; but as peculiar conveniences are required or the col- 

 lection of zoophytes outside of the line of breakers, we have 

 not the facts necessary for an exact list of such species. From 

 the very abundant masses of Astrasas, Mseandrinas, Porites, 

 and Madrepores thrown up by the waves on the exposed reefs, 

 it was evident that these genera were well represented in the 

 outer seas. In the Paumotus, the single individuals of Porites 

 lying upon the shores were at times six or eight feet in diam- 

 eter. Around the Duke of York's Island the bottom was ob- 

 served to be covered with small branching and foliaceous 

 Montipores, as delicate as any of the species in more protected 

 waters. 



Species of the same genera grow in the face of the breakers, 

 and some are identical with those that occur also in deeper wa- 

 ters. Numerous Astrseas, Mseandrinas and Madrepores grow 

 at the outer edge of the reefs where the waves come tumbling 

 in with their full force. There are also many Millepores and 

 some Porites and Pocillipores in the same places. But the 

 weaker Montipores, excepting incrusting species, are found in 

 stiller waters either deep or shallow. 



