508 Bulletin American Museum of Natural History [Vol. LIIT 
(3) In the central bowl of the Congo basin the current of the rivers 
is slow and, owing to the extremely level condition of the surrounding 
country they expand their beds considerably and often overflow 
densely forested banks. The whole central area is more or less of a 
morass or wooded swamp, especially in the region between the Mongala 
and Busira-T'shuapa Rivers, where the many forest tributaries such as 
the Lopori, Maringa, Ikelemba, ete., are connected at high water by side 
channels. In some places the banks of these rivers are so low that the 
natives are compelled to raise their huts on piles. The water of these 
affluents carries much humus and decaying vegetable matter, giving it a 
chocolate-brown or black color, even noticeable when seen in small 
quantity—for instance, in a tumbler. The water of the Congo itself is 
usually much clearer, a pale brown or greenish yellow, and a sudden 
change to a darker shade discloses the approach of the mouth of one of. 
these equatorial tributaries which is often hidden in the maze of islands 
of the main stream. Nothing is known at present of the malacological 
fauna of these equatorial forest streams, but, owing to the heavy propor- 
tion of organic matter, they probably nourish few if any mollusks. 
Between the equator and 3° S., in about 18° to 19° E., there are two 
small lakes which may be regarded as the remnants of a much larger 
body of fresh-water that covered the major part of the central bow] at a 
comparatively recent period (probably during most of the Tertiary and 
perhaps even during the Pleistocene). Both lakes are very shallow and 
bordered by flat, marshy and frequently inundated shores. The largest, 
Lake Leopold II, covers an area of about 2,300 square kilometers. Lake 
Tumba is much smaller, some 1,200 square kilometers in extent. The 
mollusks which may inhabit them are thus far unknown, but they are 
probably few in number and present nothing peculiar. 
Attention has been called above to the lacustrine conditions prevail- 
ing on the Congo River in the equatorial reach, beween the confluence 
of the Aruwimi and that of the Sanga. Extending as they do over a very 
broad but shallow bed, the waters lose much of their carrying power for 
eroded material so that silt deposits actively, forming mud banks and sand 
spits, which eventually are transformed into islands. At first it might be 
supposed that these waters, quietly flowing over a soft, muddy bottom, 
would offer almost ideal conditions for fluviatile molluscan life. One 
soon learns, however, that such is far from being the case. In fact, this is 
the part of the Congo drainage which is poorest in mollusks. Occasionally 
one may meet a mud bank where some of the Mutelide are extremely 
abundant, especially Aspatharia wissmanni bangalorum and Mutela 
rostrata. | 
