, CORAL ISLANDS. 199 
of the Jardin des Plantes, the edge of the reef is produced by 
Madrepora corymbosa, M. pocillifera, and two species of Astrea, 
which pursue 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 Seréafofora. 
At twenty fathoms we also meet with fragments of Jfadrepora. Be- 
tween twenty and forty fathoms the bottom is sandy, and the sounding- 
rod brings up great fragments of Caryophyllia. According to MM. 
Quoy and Gaimard, the species of Astrea, 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 Madregores and Seriato- 
pores down to twenty fathoms. Considerable masses of AMeandrina 
have been observed at sixteen fathoms; anda Caryophylia 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 Gorgonia at 160, Corallines at 100, Millepora at from thirty 
to forty-five, Sertularians at forty, and Zubupora at ninety-five 
fathoms. 
According to Dana, none of the species of the genera which form 
reefs—namely, Madrepora, Millepora, Poritss, Astrea, and Meandrina— 
can live at a greater depth than eighteen fathoms. It is only near 
the surface of the water that the zoantharia which produce polypidoms 
and form coral 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 finest specimens of the genera Astrea, 
Porites, and Millepora 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 accessory 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. The 
sands, too, 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 and other organic débris will probably represent a 
fourth of the total produce in relation to corals. In this manner, 
taking everything into account, the mean increase of a reef cannot 
