1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 558 
Before reaching its mouth the Congo forms a broad and deep estuary, 
about 130 kilometers long, which begins below the rapids of Kasi, some 
5 kilometers above Matadi. The waters here’ do not merely follow the 
thalweg of a simple erosion valley, but they evidently fill a deep fracture 
gorge. The stream is on the average 70 m. to 125 m. deep, but the great- 
est depth is found between Malela and Banana where a sounding of 280 
m. has been reached; right opposite Matadi the river rapidly attains a 
depth of from 50 to 70m. The gully is continued into the open sea for 
over 160 kilometers with depths as much as 1,200 m. below the general 
level of the sea floor. This fracture of the earth’s crust follows the 6th 
southern parallel and is supposedly the western continuation of the fault 
which we have mentioned above as causing the line of rapids and falls 
of the Kasai and its affluents between 5° and 6° S. The lower depths of 
this trough-like bay are occupied by nearly motionless sea water on top 
of which the fresh-water, being lighter, flows with increasing velocity, 
but decreasing depth; just within the mouth there are only a few feet of 
fresh-water. | pet 
Between Matadi and Boma the Congo still flows in one deep, wind- 
ing channel, 640 m. to 2 kilometers wide, between low hills, often with 
shallow, immersed stony ledges near the steep, rocky banks. The mala- 
cological fauna of this stretch, which is about 30 kilometers long, is 
practically unknown; yet the discovery of the peculiar littorinid snail 
Pseudogibbula duponti Dautzenberg on the immersed rocks near Vivi 
renders an investigation of this part of the river quite desirable. 
A little above Boma, the Congo enters much lower country, where it 
broadens into a huge, delta-like expansion with a number of islands. The 
first to be met with, Prince’s Island, Rocca, Sacra Baka, and Selonge, 
in the near vicinity of Boma, are rather high and hilly and contain a 
rocky core. Just below the fort of Skinkakasa, the river forms one broad 
expanse not interrupted by islands, the two banks being here at a distance 
of 4,720 m. Below that point, however, it widens again to a labyrinth 
of flat, alluvial islands, that of Mateba being the most considerable (32 
kilometers long and 4 to 7 kilometers wide). At certain points the distance 
between the right and left banks of the river reaches 18 kilometers. Some 
of the branches are very wide, but they are frequently interrupted by 
sand-bars and mud-flats. Zambi is situated about mid-way in this 
stretch, on the slightly hilly right bank of the river; the northern channel 
of the stream, between Mateba and the mainland, being here obstructed 
by a multitude of banks and alluvial, swampy islands. At Ponta da 
Lenha. about 45 kilometers from the ocean, the character of the low banks 
