1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 567 
certain fluviatile mollusks having adapted themselves to increasing 
salinity and other, marine species gradually invading less saline water. 
For it must be remembered that the amount of sea-water that enters the 
estuary 1s considerably larger at high tide, when it is also allowed to 
reach much farther upstream. The seasonal variations of salinity, due 
to the rise and fall of the Congo River, are more gradual, yet not without 
significance. The animals and plants of the brackish estuarine zone 
must therefore not merely belong to the so-called ‘“euryhaline” type of 
Meebius, which includes organisms that support fairly well a gradual 
change of salinity. Of much greater importance is the ability of adjust- 
ing their metabolism to frequent and often very rapid changes in the 
concentration of dissolved salts.! As Pelseneer has pointed out, the adap- 
tation of marine animals to brackish conditions is most easily achieved in 
the juvenile or larval stage, which is usually mobile and numerous. 
The penetration of these early stages in water of decreasing salinity is 
aided by the influx of the high tide, which thus has a mechanical action 
upon the distribution of the fauna within the estuary. 
Fluviatile Mollusks of the Congo Basin 
The following list contains only the forms of mollusks that are 
known with certainty to occur in the hydrographic basin of the Congo, 
in stagnant as well as in flowing water. The faunz of Lakes Moero, 
Kivu, and Tanganyika, however, are not included, since they are so 
distinctive as to require separate treatment. The very few species that 
have not yet been found within the political boundaries of the Belgian 
Congo are marked with an asterisk. That their number is so small may 
be in large part due to insufficient collecting. We should not expect 
many additions from the basins of the Lobay, Sanga, Likuala, Alima, 
and various other tributaries that enter the right bank of the Congo and 
Ubangi in French Equatorial Africa, ecological conditions being extremely 
similar there to those of the Upper and Middle Congo. The Malagarazi 
and Chambezi Rivers, and the many smaller eastern affluents of Lakes 
Moero and Tanganyika, in Tanganyika Territory and Northern Rhode- | 
The conditions under which marine animals adapt shempelyee to ootuntite life, and fluviatile species 
ish water, were studied by several authors. See among others: 
te reins Ae 1892. ‘On the origin of the genera of land and fresh-water Mollusca.’ Conchologist, 
. 41-48. ; : ‘ 
" grat P. 1906. ‘L’origine des animaux d’eau douce.’ Bull. Ac. Belgique, Cl. Sci., (1905), 
pp. 699-740. 
