1927] | Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 489 
of short duration. For example, at Eala, under the equator in about 18° 
28’ E., the total rainfall in 1911 amounted to 80.83 inches (2,055 mm.); 
the driest months being January with 3.24 inches (82.4 mm.) of rain and 
August with 2.69 inches (68.4 mm.); the wettest, February with 10.6 
inches (255.6 mm.), September with 12.92 inches (328.2 mm.), and 
October with 13.43 inches (341.3 mm.). The difference between the mean 
temperature of the hottest and coldest month was about 2° C. (April; 
26° .12 C.; August, 24° C.). The highest temperature registered in the 
shade was 38° .7 C., the lowest, 16° .7 C. 
The forest belt grows practically everywhere on a subsoil of hori- 
zontal sandstones and shales of Permo-Triassic age, so that limestone is 
scarce and of quite local occurrence. Only in its northeastern corner 
does it transgress over the peripheral rim of Archean and crystalline 
rocks. As much of this district occupies the flattened bowl or Central 
Basin of the Congo, it is a generally level country of low altitude, on the 
average 500 m. or less above sea-level. In the eastern portion it rises 
gradually and becomes more hilly, forming there the foothills of the 
western scarp of the Albertine Rift. It stops, however, at the 1,500 m. 
contour line, since all regions above that altitude in the eastern Belgian 
Congo belong zodgeographically to the Eastern Montane District. 
Except along certain low river banks and in the clearings made by 
the natives, the country is covered with typical forest of the moist, 
tropical, evergreen type. The chief characteristics of such a forest are 
the extreme density and endless variety of the woody vegetation, some 
of the trees attaining a considerable size, their height often exceeding 
35 m. and even occasionally reaching 50 m. Below the canopy of higher 
trees numerous bushes and young trees fill all available space, while the 
soil itself is covered with marantaceous reeds, ferns, and low herbaceous 
plants. Grasses are altogether absent or represented by a few peculiar, 
broad-leaved, heliophobic types. Many of the herbaceous plants have in 
search of light left the soil to live as epiphytes along the bole and in the 
crown of the trees. | 
Three main ecological types of forest may be distinguished in the 
lowlands of the Congo, namely, inundated forest, virgin or primary 
forest on dry land, and second-growth forest. 
The inundated forest (Pl. XLVIII) is the type with which one 
becomes first acquainted upon entering the Congo from its estuary, for 
in the Middle and Upper Congo the banks of the main stream and its 
affluents are usually very low and ill-defined, especially at high water, 
when much of the surrounding country is inundated. Thus great areas 
