1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 549 
hardly be any doubt that there has been a gradual regression of super- 
ficial water in northwestern Africa, the region of Lake Chad and. central 
South Africa, while the accompanying climatic changes certainly affected 
hygrographic conditions in the remainder of the continent as well. 
‘This happened at intervals, not only at the close of the last Pleistocene 
pluvial period, but even later and within historic times. 
That quite recently more humid conditions were general in Africa is 
of considerable zodgeographic importance since it rendered possible 
migrations of fresh-water mollusks since the Pleistocene, that is, after the 
species were mostly differentiated in their present shape. There are many 
regions where the divides between the hydrographic basins run through 
old peneplains, so much degraded to near-level that the watersheds 
are hardly defined. The river systems are frequently separated by 
but a few feet of difference in level, while at times they are in direct 
communication either by means of seasonal swamps (as in the region of 
Lake Dilolo, where the upper Kasai is connected with the upper Zambezi) 
or even through a regular channe! (asin the upper Benue, where a channel! 
to the Logone, an affluent of the Chari, is at times passable for canoes 
and leads from the basin of the Niger into that of Lake Chad). Assum- 
ing that rains were more abundant at the beginning of the modern epoch, 
such connections between the river systems must have been at times 
quite frequent and extensive, giving ample opportunity to the mollusks 
of migrating from one basin to another. : : 
Enough has -been said to make it clear that, if in the following 
account we treat in succession the various river systems of the Belgian 
Congo, it is not because we regard them as faunistic entities. It merely 
appears to be the most convenient method of presenting the available 
data in orderly fashion. 
By far the greatest part of our territory lies within the hydrographic 
basin of the Congo. The waters of a narrow, but important strip of 
territory, in the extreme northeast, flow to the Nile; while most of those 
from the western half of the Lower Congo drain directly into the Atlantic. 
Coastal Rivers of the Belgian Congo 
The most important of these, the Shiloango River, forms the north- 
ern boundary of the Belgian Congo from Mt. Kiama (in about 13° 20’ 
E.) to the confluence cf the Lubuzi, then flowing across the Portuguese 
'This leaves open the much-discussed question as to the cause of the regression of superficial water. 
It has been claimed that it is actually due to a change in climate. Other authorities claim that it can be 
sufficiently accounted for by the gradual draining off through coastal effluents of the inland lakes, while 
deforestation, greatly hastened through the agency of man, reduces the quantity of rain-water that seeps 
through the soil. 
