©. CROSSLAND—PHYSICAL DESCRIPTION OF KHOR DONGONAB. 283 
that in these places the water is far dirtier than elsewhere—that is to say, 
inside Dongonab Harbour and Khor Atof, besides in the North Basin (see 
Map 2, Pl. 29). As for other organisms, familiarity with these areas and 
watching the results of the pearl-divers shows clearly that the only organisms 
that can oceur in any numbers upon them are those which have well-protected 
larvee. For instance, among the Lamellibranchiata, all the Aviculidee, so 
common elsewhere, are rare here, being only found where they had some 
shelter during the youngest stages. Once above a diameter of 10 em. the 
large pearl-oyster (Mytilus margaritifera) lives and grows well here, where its 
newly settled larva cannot exist. The Lamellibranchiata that flourish here are 
such heavy shells as Chama, Spondylus, and Ostrea ; and it is to be noted that 
in these species the smallest shells seen, say 2 mm. across, are already hard 
and solid, in very marked contrast to the extreme delicacy of the Aviculidee 
of the same size. Similarly seaweeds except Lithothamnia are generally 
absent ; the Phyllospongiidee, which occupy the ground, may be presumed, like 
other sponges, to have larvee well provided with a skeleton, in this case of 
tough fibre, at the time they settle. 
In 1902 I suggested * that the absence of corals from the outer reef-edges 
of Zanzibar and British Hast Africa was due to the fouling of the water by 
sand and organic matter from the broad reef-flats. My experience of Khor 
Dongonab leads me to believe that the amount of sand carried out from the 
Hastern reefs of Zanzibar, and the strength of the current which impinges 
upon them, is much more than sufficient to account for the absence of coral, 
for which at that time the explanation I offered appeared to me not altogether 
adequate. 
Extension of Lagoons.—f rom the foregoing itis evident that one must never 
suppose that the existence, even in fair abundance, of Corals and Lithothamniu 
necessarily indicates the extension by growth of a reef. The rate of increase 
of a reef is not merely the sum of the growths of its individual colonies, but is 
a balance between growth and forces of degradation. Coral is quite common, 
for instance, about the base of a wall, built of corals from the reef, near the 
salt works. This has been in existence about twelve years, and whiie the 
solution of its base is very evident, the addition made by the coral colonies 
growing upon it is practically mil. Similarly on the outer reefs about 
Dongonab all dead coral is found to be rotten with sponge and worm borings, 
rapidly crumbling into mud, while the bases of even living colonies are 
generally in the first stage of decay. lor instance, a fragment of Porites 
picked up at random from a pile of coral brought in for building purposes, 
* “The Coral Reefs of Pemba Island and of the East African Mainland,” Proc. Camb, Phil. 
Soe. vol. xii. part i. (1903) pp. 86-43; and “Coral Reefs of Zanzibar,’ Proc. Camb. Phil. 
Soc. vol. xi. (1902) pp. 493-505. 
LINN. JOURN.—ZOOLOGY, VOL, XXXI. 23 
