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The Porites resemble the Astraeans, but the pits are smaller, with fewer partitions and fewer tentacles, and their whole substance is more porous. But these also have their bounds within the sea: they in their turn reach the limit beyond which they are forbidden by the laws of their nature to pass, and there they also pause.
It has been a question with some naturalists, which part of a reef is most favourable to the growth of coral. The great mounds of living Porites and of Millepora round Keeling atoll occur exclusively on the extreme verge of the reef, which is washed by a constant succession of breakers; and living coral nowhere else forms solid masses. Many other facts having a similar tendency might be adduced.
On the beach within the reef, the rolled fragments consisted chiefly of the corals just mentioned, and of a massive Porites, like that at Keeling atoll, of a Meandrina, Pocillopora verrucosa, and of numerous fragments of Nullipora.
These are the Mandrinas, or the so-called Brain-Corals, and the Porites. The Mandrinas differ from the Astraeans by their less compact and definite pits. In the Astraeans the place occupied by the animal in the community is marked by a little star-shaped spot, in the centre of which all the partition-walls meet.
It is very unusual to meet with an animal belonging to the family of Lepades in tubular holes made in the coral rocks, as is the case with the Lithonaetta N. Among the twenty kinds of tabular coral here observed, there was not one of those collected at O Tahaiti; there were three new Distichoporæ, Seriatipora, six kinds of Madrepora, two Porites, four Astrea, Pocillopora cærulea, and another kind, forming broad, yellow, leafy masses, the slime of which stings like a nettle; Cariophyllæa glabrescens Cham., and Tubipora, with red animalculæ.
In my description of Keeling atoll, I have given some facts, which show that the reef probably has grown outwards; and I have found, just within the outer margin, the great mounds of Porites and of Millepora, with their summits lately killed, and their sides subsequently thickened by the growth of the coral: a layer, also, of Nullipora had already coated the dead surface.
Almost the only natural condition, allowing a quick upward growth of the whole surface of a reef, would be a slow subsidence of the area in which it stood; if, for instance, Keeling atoll were to subside two or three feet, can we doubt that the projecting margin of live coral, about half an inch in thickness, which surrounds the dead upper surfaces of the mounds of Porites, would in this case form a concentric layer over them, and the reef thus increase upwards, instead of, as at present, outwards?
Madrepores: twelve tentacles, alternating in six larger and six smaller ones; frequently a larger top animal standing prominent in the whole community, or on the summit of its branches. Porites: twelve tentacles, not alternating in size; system of connected beams.
We shall presently see that there are other organic productions, fitted to bear a somewhat longer exposure to the air and sun. Next, but much inferior in importance to the Porites, is the Millepora complanata.
Between these plates and in the protected crevices on the reef, a multitude of branching zoophytes and other productions flourish, but 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.
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