1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 463 
the fact that humidity at first increases with altitude, while at higher — 
levels the lower atmospheric pressure produces a drier atmosphere, but 
the orientation of the slopes too is of great importance in deciding whether 
they will be exposed to dry or wet winds. Other climatic factors, such 
as intensity of sunlight and wind, are hardly of importance in the life of 
mollusks and need not here be discussed. 
The nature of the vegetation depends directly upon climatic condi- 
tions and, in tropical Africa, especially upon the total amount and 
seasonal distribution of rainfall. In the Congo lowlands the dominant 
plant formations belong to one of two main ecological types: rain forest 
and savanna. RAIN FOREST, as here understood, is any plant formation 
of dense, continuous growth chiefly composed of trees and bushes, while 
grasses are elther entirely absent or restricted to. a few, heliophobic or 
light-shunning types. Likewise, the term SAVANNA is meant to cover all 
plant formations where grasses predominate, whether there be or be not 
an admixture of shrubs, bushes, or trees. Jain forest in tropical Africa 
thrives wherever the rains are abundant enough and so evenly distri- 
buted over the year as to allow an uninterrupted development of woody 
plants. Savanna covers regions with sharply marked and prolonged dry 
seasons, during which most of the plants stop growing and many of the 
trees even drop their leaves. These two types of tropical vegetation affect 
molluscan life in a very different manner. In the rain forest the steady 
moisture, dense shade, and luxuriant vegetation allow land mollusks to 
thrive throughout the year and consequently this is the domain of the 
large, hygrophilous snails and of most of the slugs. Species which live 
in the savanna, on the other hand, must be able to bridge over the dry 
season in a dormant state. That mollusks are very few in species and 
individuals in the African grass-lands is due perhaps not so much to 
unfavorable ecological conditions as to the grass fires which every dry 
season sweep over the country (compare Pls. LII and LIII). Only 
those snails and slugs can survive that in order to estivate have dug in 
sufficiently deep or are hidden in crevices of rocks or in hollow branches 
or stumps of trees. In the Central African mountains the altitudinal 
distribution of plant formations, resulting in a corresponding succession 
of molluscan life-zones, is of a more complex nature, as we shall describe 
for some of the mountain ranges of the eastern Congo. | 
Soil conditions perhaps influence more directly the distribution and 
development of land mollusks than any other single factor and they are 
certainly of much greater importance to them than to most other 
