1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 555 
and of foreign substances dissolved or held in suspension. When’ the 
waters suddenly rise, islets of floating vegetation may become detached 
from the banks and be carried downstream together with drift ; and 
snails and mussels, or their eggs, may thus be spread over great distances. 
As the basin of the Congo extends far to the north and to the south 
of the equator, where, as we have explained above (p. 461), the rainy 
and dry seasons occur at different times of the year, the lower part of the 
Congo River has two seasons of greatest flood. Above the confluence 
of the Kasai, these come in May and November, that is to say, toward 
the end of the southern and northern rainy seasons. The Lualaba above 
Stanley Fallsis swollen during March and April, at the end of the rainy 
season in Katanga, and the difference of level with the lowest water is 
quite considerable, since in Upper Katanga the dry season (May to 
September inclusive) is extremely pronounced. The highest waters in 
the northern tributaries (Uele, Bomu, Ituri-Aruwimi, etc.) are observed 
in October-November; on the lower Ubangi somewhat later (December). 
The flood of the Kasai basin corresponds to that of the upper Lualaba 
(March-April). Its great volume of water, carried more directly to the 
Lower Congo than is the drainage of Katanga, tends to accelerate and to 
lengthen one of the periods of flood in Stanley Pool and below, where the 
highest levels are reached about December and April-May. At Stanley 
Pool the maximum rise of the water is about 5 m., but it varies much from 
year to year. Below the Cataracts, at Matadi, the difference in level 
between high and low water is still greater, 6 to 8 m. Farther down- 
stream it rapidly decreases, since it is neutralized by the action of the 
tides. At Bomait amounts to 3 m.; at the western extremity of Mateba 
Island to 1.8 m.; and at Mateba to only 0.9m. Yet the difference in the 
volume of fresh-water carried by the river at the several seasons is such 
that it must profoundly modify the degree of salinity in the estuary. 
Even within the zone of brackish water the animals and plants are 
forced to adjust their metabolism to frequent and rather rapid changes in 
the concentration of mineral salts. With regard to the smaller tributaries 
of the Congo, it is generally true that the farther from the equator is the 
country they drain, the greater will be the difference between high and 
low water. 
From the foregoing general description it appears that the flowing 
waters of the Congo basin belong to four ecological types, each of which 
possesses a number of peculiar mollusks. : 
(1) The brooks and streams of the headwaters are small, often but a 
few inches deep and moderately swift. They generally flow over a 
