120 CORALS AND CORAL ISLANDS. 



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

 reef inside of the breakers. Astrseas, Masandrinas 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 Hon- 

 den Island (Paumotus), all others having disappeared, owing 

 to its imperfect connection with the sea. Upon the reefs en- 

 closing the harbor of Rewa (Viti Lebu), where a large river, 

 three hundred yards wide empties, which during freshets en- 

 ables vessels at anchor two and a half miles off its mouth to 

 dip up fresh water alongside, there is a single porous species 

 of Madrepora (M. cribripora), growing here and there in 

 patches over a surface of dead coral rock or sand. In similar 

 places about other regions, species of Porites are most com- 

 mon. In many instances, the living Porites were seen stand- 

 ing six inches above low tide, where they were exposed to sun- 

 shine and to rains ; and associated with them in such exposed 

 situations, there were usually great numbers of Alcyonia and 

 Xeni. Theaa Siderinaa endure well exposure to the air. 



The exposure of six inches above low tide, where the tide 

 is six feet, as in the Feejees, is of much shorter duration than 

 in the Paumotus, where the tide is less than half this amount ; 

 and consequently the height of growing coral, as compared 

 with low-tide level, varies with the height of the tides. 



Porites also occur in the impure waters adjoining the 

 shores ; and the massive species in such places commonly 

 spread out into flat disks, thy top having died from the depo- 

 sition of sediment upon it. 



The effects of sediment on growing zoophytes are strongly 

 marked, and may be often perceived when a mingling of fresh 

 water alone produces little influence. We have mentioned 

 that the Porites are reduced to flattened masses by the lodg- 



