There are other constituents of coral reefs of not very much less importance than the corals themselves. Large masses are formed of the bivalve shells which live in the coral mud, and which, by the solidification of this mud, form with it a limestone, such as that of which the houses of Port Sudan are built. Although among the very numerous and conspicuous fossils of this stone coral branches are not the commonest, yet the mass of shells and hardened mud is every bit as much a part of the reef as anything else is. In some reefs too, even where coral is growing abundantly, the shells of the great clam _Tridacna_ are so abundant as to make up a considerable part of the total mass. Others again contain quantities of certain peculiar seaweeds (of which the “coralline” of British seas is one) which, though true plants in every detail, have the property of taking up limestone from the sea and forming therewith a skeleton, even harder and more compact than that of the corals. Plate XXX shews the appearance of these plants, and will enable the reader to identify some of those he meets with. These sometimes form a cement, by which the coral colonies and fragments are held together, and in some others the whole reef is formed of them[47]. Other organisms assist, but I observe my principle of dealing only with the most important features and desist from enumerating all. This is the whole structure of the interior of such a reef as that which fringes the Red Sea coast, as seen e.g. during the excavation of the quay walls or slipway at Port Sudan. Great “stones” which are the more massive colonies, generally the genus _Porites_, are bedded in with smaller colonies whole or broken. In places are collections of grey mud and sand, also formed from coral by the action of boring organisms or perhaps as the residue left after partial solution by the sea. “How fast does a coral reef grow?” is a question often asked, and never as yet truly answered. Probably each of the hundreds of species of coral has its own maximum rate of growth, which is however rarely attained, as it is certain that the rate of every colony of each species varies very widely with its position on the reef and its immediate surroundings. So taking the rate of growth of a few samples would go a very little way towards giving that of the corals in any given square yard of the reef edge. Although individual colonies may grow quite rapidly this is but half the question. We must know also full details of the action of eroding and transporting sea currents, solution, boring organisms (in coral, coral sand and mud) and subtract the total from that of the deposition of stone by living polyps, to obtain the net increase. _Plate XXX_ [Illustration] [Illustration: Figs. 64 and 65. Stony seaweeds, massive and branched. _Lithothamnia_]