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Fragments of the Millepora alcicornis and of an Astraea were also numerous; the former is found, but not in proportionate numbers, in the hollows on the reef; but the Astraea I did not see living.
Millepora alcicornis lives in from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0 to 20 fathoms. Mr. SERTULARIA, 40, Cape Horn 66. CELLARIA, 40, Cape Horn 66. CELLARIA, A minute scarlet encrusting species, found living, 190, Keeling Atoll, 12. CELLARIA, An allied, small stony sub-generic form, 48, St Cruz Riv. 50.
Besides these lithophytes, some fragments of the Millepora alcicornis, which occurs in the same relative position at Keeling Island, were brought up; and in the deeper parts there were large beds of a Seriatopora, different from S. subulata, but closely allied to it.
The corals, also, on the margin of Keeling Island occurred in zones; thus the Porites and Millepora complanata grow to a large size only where they are washed by a heavy sea, and are killed by a short exposure to the air; whereas, three species of Nullipora also live amidst the breakers, but are able to survive uncovered for a part of each tide; at greater depths, a strong Madrepora and Millepora alcicornis are the commonest kinds, the former appearing to be confined to this part, beneath the zone of massive corals, minute encrusting corallines and other organic bodies live.
Moreover, at Keeling atoll, and probably in many other cases, the different kinds of corals would have to encroach on each other; thus the Nulliporae cannot increase outwards without encroaching on the Porites and Millepora complanata, as is now taking place; nor these latter without encroaching on the strongly branched Madreporet, the Millepora alcicornis, and some Astraeas; nor these again without a foundation being formed for them within the requisite depth, by the accumulation of sediment.
The arming of the lead here invariably came up quite clean, but deeply indented, and chains and anchors which were lowered, in the hopes of tearing up the coral, were broken. Many small fragments, however, of Millepora alcicornis were brought up; and on the arming from an eight-fathom cast, there was a perfect impression of an Astraea, apparently alive.
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