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Show 178 MR. CYRIL CROSSLAND ON THE [Mar. 7, hundreds of square yards of the coast of Pemba and the Zanzibar Channel *. 4 . T h e O r g an ic D epo sit s. Nullipores, on the other hand, are extremely abundant; every rock exposed to the surf is thickly coated with them, and since the coasts are nearly all rock, and the surf penetrates to every bay, the total amount is enormous. Owing to this nature of the shores, it is impossible to land in most parts of the Islands, so that it is not easy for a worker who is necessarily confined to the few more or less sheltered bays to obtain a correct general idea of the condition of the balance of life round the coasts as a whole. I have been able to make detailed examinations of fully exposed rocks at Bird Island at the entrance to St. Yincent Harbour and in Bonavista, and have seen sufficient of other coasts to know that these examples are typical of practically the whole coast-line. The most exposed projections of the rock on Bird Island are covered by a nullipore of a stout foliaceous kind, consisting of vertical branches connected at intervals by horizontal platforms. Between the areas occupied by this species the surface seems to be made of smooth encrusting nullipore bearing clumps of mossy green and brown weed. These form a broad belt extending from near high-tide mark to a little below the level of lowest tides. Above this bed is a zone of Balanus, while below the rock is merely painted over with nullipore. On breaking into the smooth incrustation it is almost always found to consist, not of nullipore alone, as would be concluded from its external appearance, but largely also of the shells of one of the fixed Gastropods ( Vermetus), the interstices between the coiled tubes alone being filled in by the Alga. In the partial shelter of the bays the character of the incrustation changes, as well as diminishing in thickness. The complete series of changes is well illustrated in the vicinity of Port Sal Rei, Bonavista, as one passes from the complete shelter of Pequena Island to the exposed rocks of the N.W. comer of the island. At first nullipores are practically absent, but the shore is covered by flat round stones, each of which consists of a nucleus of volcanic rock, the diameter of which has been trebled by the addition of a mass of the Vermetus round its sides. Passing northwards, where the surf begins to take effect, nullipores appear in conjunction with the Vermetus, forming a more or less smooth incrustation four inches to a foot in thickness, while outside the Pequena Channel the foliaceous species of nullipore appear as at Bird Island and other exposed coasts, and the proportion of Vermetus has greatly decreased. The mode of growth of the Vermetus results in the enclosing of spaces between its own mass and the surface of the rock, which communicate with the outer water by numerous holes and crevices. As would be expected, * Millipore is fairly common in many places in these islands, forming incrustations or sparsely-branched growths, |