499 
Mr Crossland, The Coral Reefs of Zanzibar. 
The sand is somewhat discoloured by organic matter on the reef, 
but on the beach it is almost white, and when dry the surface 
assumes the forms characteristic of that of dry powdery snow 
under the influence of the wind. 
Further north, for several miles to the south of RasMichamve,the 
sand almost disappears, except on the shore side. Here (Dingwe) 
the channel is from 4 to 8 ft. deep. On either side of it are here 
and there large flat -topped cylinders of coral (. Porites ) from 6 to 
1 2 feet each way, growing at the sides but dead at the surface 
of the water. This genus flourishes where other corals are absent, 
e.g. also in the channel at Ras Nungwe. Beyond the channel is 
a gentle slope from 2 feet of water up to the undermined cliffs 
which form the shore. 
A peculiarity of the reef to the north of Ras Michamve 1 is 
the way it is scooped out into big cylindrical basins, averaging 
12 feet across by 6 deep, which are often so close together 
that their edges form only a narrow flat winding pathway across 
the reef. They are full of clear water, which flows from one to 
another or may be stationary. I expected corals to flourish on 
their sides, but they, and most other organisms, are quite absent. 
The shore side of the reef is a long flat of barren rock coated 
with slimy mud. 
Chuaka Bay is at low springs mainly an expanse of sand- 
banks on which are large beds of Zoster a (three species) and of 
a small stiff Halimeda. The central bank is covered with large, 
flat sponges, lying unattached, which one turns over in collecting 
as one would stones. A little coral is found below low tide 
mark at the mouth of the bay. All the surface which is not 
sand or mud is bare rock, comparatively barren of organisms. 
From the Halimeda and Zoster a beds however I have collected 
about 100 species of Opisthobranchs, and the sand, but not the 
mud, is very rich in Polychaetes. 
Halimeda is a factor of no importance in rock formation here. 
The dead “leaflets” cover the surface of the beds, but remain 
whole only to the depth of an inch or so, disintegrating below 
to a fine grey mud which in exposed situations would be swept 
away at once. 
In many places north of Chuaka Bay the boat channel is more 
or less filled with coarse sand matted together by the roots of 
a dwarf Zostera. The sand is cut out into basin-like pools reach- 
ing down to the rock. This is so near Muyuni over the inner 
third of the reef flat, and to the south it forms the greater part 
of the reef surface for several miles. 
1 The reef edge is here again stony, the blocks being somewhat larger than 
I have seen elsewhere. It descends gradually below low tide level. 
VOL. XI. PT. VI. 
35 
