1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo — 551 
~ 
Nae 
_Lake Kivugg ) 
A fi 
fi 
Map 15. Hydrographic map of the Congo basin and the adjacent river systems. 
The peripheral regions of the Congo basin are generally at an alti- 
tude of over 500 m. and in the southern and eastern portions frequently 
reach 1,000 m. and more. Much of this area is a fairly level or slightly 
hilly peneplain, occasionally interrupted by high plateaus (as in Katanga) 
or by moderately high mountains (as in the Crystal Mountains of the 
Lower Congo, the Bia and Hakannson Mountains of Katanga, etc.). 
On the level peneplains the streams are sluggish, though generally flowing 
within well-defined banks; but in truly mountainous country the head- 
waters are frequently swift torrents. The central portion of the basin, 
however, approximately between 3° N. and 4°S., 16° E. and 26° E., forms 
an almost level plain or flattened bowl rarely reaching 500 m. above sea- 
level. Here the rivers broaden considerably and become sluggish and 
shallow; their course is obstructed with many sand spits and mud banks 
