CAUSES AFFECTING GROWTH OF CORALS. 93 



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

 nels, though true of lagoons and channels of small si7e, or of 

 such parts of the larger channels as immediately adjoin the 

 mouths of fresh-water streams. 



There are undoubtedly species especially fitted for the open 

 ocean ; but as peculiar conveniences are required for 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 Astrseas, 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 

 diameter. Around the Duke of York's Island, the bottom 

 was observed to be covered with small branching and folia- 

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

 ])rotected waters. 



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

 and some are identical with those that occur also in deeper 

 waters. Numerous Astraeas, 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 Miile- 

 l)ores 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. 



Again, the same genera occur in the shallow waters of the 

 reef inside of the breakers. Astraeas. Mseandrinas and Pocilli- 

 pores are not uncommon, though requiring pure waters. There 

 are also Madrepores, some growing even in impure waters. 

 One species was the only coral observed in the lagoon of 



