10 
ATOLLS. 
Cn. I. 
Porites, is the Millepora complanata. 1 It grows in 
thick vertical plates, intersecting each other at various 
angles, and forms an exceedingly strong honey-combed 
mass, which generally assumes a circular form, the 
marginal plates alone being alive. Between these plates 
and in the protected crevices on the reef, a multitude 
of branching zoophytes and other productions flourish, 
hut the Porites and Millepora alone seem able to resist 
the fury of the breakers on its upper and outer edge ; 
at the depth of a few fathoms other kinds of stony 
corals live. Mr. Liesk, who was intimately acquainted 
with every part of this reef, and likewise with that of 
North Keeling atoll, assured me that these corals in- 
variably compose the outer margin. The lagoon is 
inhabited by quite a distinct set of corals, generally 
brittle and thinly branched ; but a Porites, apparently 
of the same species with that on the outside, is found 
there, although it does not seem to thrive, and cer- 
tainly does not attain the thousandth part in bulk of 
the masses opposed to the breakers. 
The wood-cut (No. 3) shows the form of the bot- 
tom outside the reef : the water deepens very gradually 
for a space of between one and two hundred yards 
wide, to a depth of 25 fathoms (A in section), beyond 
which the sides plunge into the unfathomable ocean 
at an angle of 45° 2 . To the depth of ten or twelve 
1 This Millepora, (Palmipora of Blainville,) as well as the M. alci- 
cornis, possesses the singular property of stinging the skin where it 
is delicate, as on the face and arm. 
2 The soundings from which this section is laid down were taken 
with great care by Captain FitzKoy himself : he used a bell-shaped 
