178 MR. CYRIL CROSSLAND ON THE [Mar. 7, 



hundreds of square yards of the coast of Pemba and the Zanzibar 

 Channel *. 



4. The Organic Deposits. 



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 natvire of the shores, it is impossible to land in 

 most parts of the Islands, so tliat 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 

 sufiicient 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 laullipore of a stout foliaceous kind, consisting of 

 vertical branches connected at intervals by horizontal platforms. 

 Between the areas occupied by this species the siirface 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 levfil 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 

 fi-om its external appearance, but largely also of the shells of one 

 of the fixed Gastropods (Vennetus), the interstices between the 

 coiled tubes alone being filled in by the Alga. 



In the partial shelter of the bays the character of the incrusta- 

 tion 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. corner 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 i-ound its sides. Passing north- 

 wards, 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 propoi'tion of 

 VermeUis has greatly decreased. The mode of growth of the 

 Vei'inetus 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, 



* Millipoi-e is fairly common in mnnj' places in these islands, forming incrus- 

 tations or sparsely-branched growths, 



