120 CORALS AND CORAL ISLANDS. 
Again, the same genera occur in the shallow waters of the 
reef inside of the breakers. Astrzeas, Meeandrinas 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 
Xeniz. The Siderinz 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, the top having died from the depo- 
sition of sediment upon it. 
The effects of sediment on growing zoéphytes 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- 
