Sect. I. 
KEELING ATOLL. 
13 
indicates the probable existence of submarine cliffs 
of rock. 
On the margin of the reef, close within the line 
where the upper surface of the Porites and of the 
Millepora is dead, three species of Nullipora flourish. 
One grows in thin sheets, like a lichen on old trees; the 
second in stony knobs, as thick as a man’s finger, 
radiating from a common centre ; and the third, which 
is less common, in a moss-like reticulation of thin, but 
perfectly rigid branches . 1 The three species occur 
either separately or mingled together ; and they form 
by their successive growth a layer two or three feet in 
thickness, which in some cases is hard, but where formed 
of the lichen-like kind, readily yields an impression to 
the hammer : the surface is of a reddish colour. These 
Nulliporae, although able to exist above the limit of 
true corals, seem to require to be bathed during the 
greater part of each tide by breaking water, for they are 
not found in any abundance in the protected hollows on 
the back part of the reef, where they might be immersed 
during either the whole or an equal proportional time 
of each tide. It is remarkable that organic productions 
of such extreme simplicity, for the Nulliporte undoubt- 
edly belong to one of the lowest classes of the vegetable 
kingdom, should be limited to a zone so peculiarly cir- 
1 This last species is of a beautiful bright peach-blossom colour. 
Its branches are about as thick as crow-quills ; they are slightly 
flattened and knobbed at the extremities. The extremities only are 
alive and brightly coloured. The two other species are of a dirty 
purplish white. The second species is extremely hard ; its short 
knob-like branches are cylindrical, and do not grow thicker at their 
extremities. 
3 
