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. 



" To 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 mseandrina and astrsea, 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 reef. 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 mseandrina 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 mseandrina, astrsea, 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 se< a. 

 When the slope is more gentle, the greal groups of living corals and intervening 

 spaces of white sand can be still discerned through the char water to a depth 

 of forty or fifty feet, beyond which the water recovers its usual deep blue. 1 



1 Jukes (p. 11, lur. /it.) 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. He also states that a shoal with even five 



