CORAL-REEFS. in 



Chagos atolls, graduated into a sandy slope, it appears 

 very probable that the depth, at which reef-building poly- 

 pifers can exist, is partly determined by the extent of 

 inclined surface, which the currents of the sea and 

 the recoiling waves have the power to keep free from 

 sediment. 



MM. Quoy and Gaimard 1 believe that the growth of 

 coral is confined within very limited depths ; and they state 

 that they never found any fragment of an Astrsea (the genus 

 they consider most efficient in forming reefs) at a depth 

 above 25 or 30 feet. But we have seen that in several 

 places the bottom of the sea is paved with massive corals 

 at more than twice this depth; and at 15 fathoms (or twice 

 this depth) off the reefs of Mauritius, the arming was marked 

 with the distinct impression of a living Astraea. Millepora 

 alcicornis lives in from o to 12 fathoms, and the genera 

 Madrepora and Seriatopora from o to 20 fathoms. Capt. 

 Moresby has given me a specimen of Sideropora scabra 

 (Pontes of Lamarck) brought up alive from 17 fathoms. 

 Mr. Couthouy 2 states that he has dredged up on the 

 Bahama banks considerable masses of Meandrina from 16 

 fathoms, and he has seen this coral growing in 20 fathoms. 

 A Caryophyllia, half an inch in diameter, was dredged up 

 alive from 80 fathoms off Juan Fernandez (lat. 33 S.) by 

 Capt. P. P. King : 3 this is the most remarkable fact with which 

 I am acquainted, showing the depth at which a genus of 

 corals often found on reefs, can exist. 4 We ought, however, 



1 Annates des Sci. Nat., torn. vi. 



2 Remarks on Coral Formations, p. 12. 



3 I am indebted to Mr. Stokes for having kindly communicated this 

 fact to me, together with much other valuable information. 



4 I will record in the form of a note all the facts that I have been 

 able to collect on the depths, both within and without the tropics, at 



