| 1927] Pilsbry-Bequaert, The Aquatic M ollusks of the Belgian Congo 547 
hypothetical, we have as much as possible kept within the limits of well- 
established geologic and topographic observations. We have also kept 
in mind that these lakes must have been fed by rivers draining a suffi- 
ciently extended continental area from which the alluviums were 
Map 14. Hypothetical extent and distribution of lake and river basins on the 
African continent, toward the close of the Pleistocene. 
eroded that eventually filled them. Even so they appear rather exten- 
sive as compared with the size of the present African lakes, and pre- — 
suppose a rainfall at least four or five times as heavy as nowadays. 
These late Pleistocene lakes have been shown as occupying the lower 
portions of closed continental basins to which many of the rivers flowed 
whose waters nowadays find an outlet to the sea. Such may have been 
‘actually the case for some of them. But even when they eventually 
obtained an effluent through capture by a coastal river, these lakes may 
have retained a considerable size for some time. It is probable that their 
