134 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
depth all round, whether that depth be great or small, are more commonly of 
an oval, circular, or irregular shape, but these are usually much larger when 
exposed to the wind and surf than in more sheltered situations. 
“То get an idea of the nature and structure of an individual coral reef, let 
the reader fancy to himself a great submarine mound of rock, composed of the 
fragments and detritus of corals and shells, compacted together into a soft spongy 
sort of stone. The greater part of the surface of this mound is quite flat and 
near the level of low water, At its edges it is commonly a little rounded off, 
or slopes gradually down to a depth of two, three, or four fathoms, and then 
pitches suddenly down with a very rapid slope into deep water, 20 or 200 
fathoms, as the case may be. The surface of this reef, when exposed, looks 
like a great flat of sandstone with a few loose slabs lying about, or here and 
there an accumulation of dead broken coral branches, or a bank of dazzling 
white sand. It is, however, checkered with holes and hollows more or less 
deep, in which small living corals are growing; or has, perhaps, a large portion 
that is always covered by two or three feet of water at the lowest tides, and 
here are fields of corals, either clumps of branching madrepores, or round stools 
and blocks of mæandrina and astrea, both dead and living. Proceeding from 
this central flat towards the edge, living corals become more and more abundant. 
As we get towards the windward side, we of course encounter the surf of the 
breakers long before we can reach the extreme verge of the reef, and among 
these breakers we see immense blocks, often two or three yards (and sometimes 
much more) in diameter, lying loose upon the тесі. These are sometimes within 
reach by a little wading; and though in some instances they are found to con- 
sist of several kinds of corals matted together, they are more often found to be 
large individual masses of species, which are either not found elsewhere, and 
consequently never seen alive, (I have seen a block of meandrina of irregular 
shape, twelve or fifteen feet in diameter, the furrows of which, though much worn 
and nearly obliterated, were wider than my three fingers; also very large blocks 
and crags of a porites, twenty feet long and ten feet high, but all one connected 
mass, without any breaks in its growth,) or which greatly surpass their brethren 
on other parts of the reef in size and importance. If we approach the lee edge 
of the reef, either by wading or in a boat, we find it covered with living corals, 
commonly mæandrina, astro, and madrepore, in about equal abundance, all 
glowing with rich colors, bristling with branches, or studded with great knobs 
and blocks. When the edge of the reef is very steep, it has sometimes over- 
hanging ledges, and is generally indented by narrow winding channels and 
deep holes, leading into dark hollows and cavities where nothing can be seen. 
When the slope is more gentle, the great groups of living corals and intervening 
spaces of white sand can be still discerned through the clear water to a depth 
of forty or fifty feet, beyond which the water recovers its usual deep blue." 
1 Jukes (p. 11, (ос. cit.) says he could distinguish the bottom as deep as seven 
fathoms, but in ten fathoms the depth of color can scarcely be distinguished from 
the dark azure of the unfathomable ocean. Ile also states thata shoal with even five 
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