18 
ATOLLS. 
Ch. I. 
lying loose on the beach, had their central parts altered 
and infiltrated . 1 
The lagoon alone remains to he described ; it is 
much shallower than that of most atolls of considerable 
size. The southern part is almost filled up with banks 
of mud and fields of coral, both dead and alive ; but 
there are considerable spaces, from three to four 
fathoms, and smaller basins from eight to ten fathoms 
deep. Probably about half its area consists of sedi- 
ment, and half of coral-reefs. The corals composing 
these reefs have a very different aspect from those on 
the outside : they are numerous in kind, and most 
of them are thinly branched. Meandrina, however, 
lives in the lagoon, and many great rounded masses 
of this coral lie loose or almost loose on the bottom. 
The other most common species are three closely 
allied species of true Madrepora with thin branches ; 
Seriatapora subulata ; two species of Porites 2 with 
cylindrical branches, one of which forms circular clumps, 
with only the exterior branches alive; and lastly, a 
coral something like an Explanaria, but with stars on 
both surfaces, growing in thin, brittle, stony, foliaceous 
1 [Dead coral still lying on the beach has been found to contain 
at least 5 per cent, of carbonate of magnesia, though only a very 
small quantity is present in fresh coral (usually less than 1 per cent.). 
In old coral-rock as much as 3S 07 per cent, has been found (Dana’s 
Corals and Coral Islands, ch. vi. § 9).] 
2 This Porites has somewhat the habit of P. clavaria, but the 
branches are not knobbed at their ends. When alive it is of a 
yellow colour, but after having been washed in fresh water and 
placed to dry, a jet-black slimy substance exuded from the entire 
surface, so that the specimen now appears as if it had been dipped 
in ink. 
