90 CONDITIONS FAVOUEABLE TO Ch. IV. 



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 Ma- 

 drepora 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. If we com- 

 pare the external margin of the reef at Keeling atoll 

 with that on the leeward side of Mauritius, which are 

 very differently circumstanced, we shall find a corres- 

 ponding difference in the appearance of the corals. At 

 the latter place, the genus Madrepora is preponderant 

 over every other kind ; and beneath the zone of massive 

 corals there are large beds of Seriatopora. There is 

 also a marked difference, according to Captain Moresby, 1 

 between the great branching corals of the Red Sea, and 

 those on the reefs of the Maldiva atolls. 



These facts, which in themselves are deserving of 

 notice, bear, perhaps, not very remotely on a remarkable 

 circumstance which has been pointed out to me by 

 Captain Moresby, namely, that with very few exceptions, 

 none of the coral-knolls within the lagoons of Peros 

 Banhos, Diego Grarcia, and the Great Chagos Bank (all 

 situated in the Chagos group), rise to the surface of the 

 water ; whereas, with equally few exceptions, all those 

 within Solomon and Egmont atolls in the same group, 

 and likewise those within the large southern Maldiva 

 atolls, reach the surface. I make these statements, after 



1 Captain Moresby on the Northern Maldiva Atolls, Geograph. Journ. 

 vol. v. p. 401. 



