1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 581 
Lake Tanganyika 
Lake Tanganyika lies between 3° and 9° S., in about 29° to 31° E. 
It is the longest fresh-water lake in the world, extending from southeast 
to northwest over some 700 kilometers, while its width varies between 
40 and 80 kilometers. It covers an area of approximately 32,900 square 
kilometers and occupies, at an altitude of 771 m., the southern end and 
deepest portion of the Albertine Rift. Almost everywhere the coast-line 
rises abruptly to a considerable height, frequently forming sheer cliffs. 
In many places the shore is rocky, but elsewhere there are beaches of 
pebbles or coarse sand on which dead shells are found in profusion (PI. 
LXX). Some ofthe rivers flowing into the lake have also accumulated 
much silt at their mouth and the marshy shore is then fringed with dense 
thickets of reed (Phragmites vulgaris). Most of the tributaries, however, 
are small; the Malagarazi, on the eastern shore, which drains a large part 
of Tanganyika Territory, is by far the most important. In the extreme 
north the Ruzizi, coming from Lake Kivu, enters by several branches 
which form the delta of Ugende, a low, alluvial, swampy plain evidently 
once a portion of the floor of Tanganyika. About the middle of the 
western shore, in nearly 6° S8., there is a break through the mountainous 
scarp, giving passage to the Lukuga, the outlet of Tanganyika to the 
Congo. There are very few islands, always of small size. In the north- 
western corner the mountainous peninsula of Kibanga, 60 kilometers long 
and 15 kilometers wide, separates the broad and deep Burton Gulf from 
the main body of water. 
The history of the Lukuga River is extremely interesting and not 
without importance from a malacological point of view, as it would appear 
to have had many vicissitudes. Before the final subsidence which pro- 
duced the deep rift of which Tanganyika now fills the bottom, the primi- 
tive valley was subjected to active erosion by a stream which either 
drained a now vanished surface of land or was fed by a precursor of the 
present lake. This stream gradually scooped out its bed through the 
more recent continental formations and down to the older Palzeozoic 
rocks. The formation of Lake Tanganyika in its present shape cut off 
the main source of supply of the Lukuga and during the period the lake 
filled up to its present level, alluvial sediments gradually choked the bed 
of the stream. Since the area drained by Tanganyika is not very 
great and rainfall relatively low, the amount of water which found its 
way into the lake may have been at certain periods nearly balanced by 
evaporation, so that there was little change in the surface level and no 
occasion for an outlet. If the waters did not flow out at all for some 
