258 CORALS AND CORAL ISLANDS. 
corals, except rarely some small tufts. If they indicate any 
thing, it must be the growth of the reef-rock, and not of the 
corals themselves. But the shore-platform where they are 
found is not increasing in height; its elevation above low-tide 
being determined, as has been shown, by wave action (page 
232). They resemble, in fact, other saxicavous mollusks, sev- 
eral species of which are found in the same seas, some 
buried in the solid masses of dead coral lying on the reef. 
The bed they excavate for themselves is usually so complete 
that only an inch or two in breadth of their ponderous shells 
are exposed to’view. Without some means like this of secur- 
ing their habitations, these mollusks would be destroyed by 
the waves; a tuft of byssus, however strong, which answers 
for some small bivalves, would be an imperfect security against 
the force of the sea for shells weighing one to five hundred 
pounds. vn 
IV. ORIGIN OF THE BARRIER CONDITION OF REEFS, AND 
OF THE ATOLL FORMS OF CORAL ISLANDS. 
I. OLD VIEWS. 
In the review of causes modifying the forms of reefs, no 
reason is assigned for the most peculiar, we may say the most 
surprising, of all their features,—that they so frequently take 
a belt-like form, and enclose a wide lagoon; or, in other cases, 
range along, at a distance of some miles, it may be, from 
the land they protect, with a deep sea separating them from 
the shores. 
This peculiar character of the coral island was naturally 
the wonder of early voyagers, and the source of many specu- 
lations. The instinct of the polyp was made by some the sub- 
ject of special admiration; for the “helpless animacules” 
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