] 76 THE OCEAN WORLD. 



their operations at the depth of from eight to fifteen fathoms. At the 

 base is a bank of Seriatopora, from fifteen to twenty fathoms in 

 height. At the bottom, the sand is covered with Seriatopora. At 

 twenty fathoms we also meet with fragments of Madrepora. Between 

 twenty and forty fathoms the bottom is sandy, and the sounding- rod 

 brings up great fragments of Caryophylla. According to MM. 

 Quoy and Gaimard, the Astreas, which, as these naturalists consider, 

 constitute the greater part of the reefs, cannot live beyond four or five 

 fathoms deep. Millepora alcicornis extends from the surface to the 

 depth of twelve fathoms ; the Madrepores and Seriatopores down to 

 twenty fathoms. Considerable masses of Meandrina have been ob- 

 served at sixteen fathoms ; and a Caryophylla has been brought up 

 from eighty fathoms in thirty- three degrees south latitude. Among the 

 polyps which do not form solid reefs, Mr. Darwin mentions Cellaria, 

 found at a hundred and ninety fathoms deep, Gorgonia at a hundred 

 and sixty, Corallines at a hundred, Millepora at from thirty to forty- 

 five, Sertularias at forty, and Tubulipora at ninety-five fathoms. 



According to Dana, none of the species which form reefs namely, 

 Madrepora, Millepora, Forties, Astreas, and Meandrineas can live 

 at a greater depth than eighteen fathoms. It is only near the surface 

 of the water that the zoophytes which produce minerals and form 

 madreporic banks put forth their powers ; the points most exposed to 

 the beating of the waves is that which is most favourable to their 

 growth ; it is there that the Astreas, Porites, and Millepores most 

 abound. 



The proportionate increase of the structures, according to Mr. 

 Darwin, depends at once upon the species which construct the reefs 

 and upon various accessary circumstances. The ordinary rate of in- 

 crease of the madrepores, according to Dana, is about an inch and a 

 half annually ; and, as their branches are much scattered, this will not 

 exceed half an inch in thickness of the whole surface covered by the 

 madrepore. Again, in consequence of their porosity, this quantity 

 will be reduced to three -eighths of an inch of compact matter. It is, 

 besides, to be noted that great spaces are wanting ; the sands filling 

 up the destroyed part of the polyp are washed out by the currents in 

 the great depths where there are no living corals, and the surface 

 occupied by them is reduced to a sixth of the whole coralline region, 

 which reduces the preceding three-eighths to one-sixth. The shells 



