1927] Pilsbry-Bequaert,-The Aquatic Mollusks of the Belgian Congo 49} 
ate, very thin, and vitrinoid. The Enide are only represented by a few 
species of Pachnodus. The Vaginulide, though abundant in individuals, 
show but little specific diversity and the Urocyclide appear to be con- 
spicuously rarer than in the montane forest of the eastern highlands. 
Striking negative features are the almost total absence of true Helicidee 
(one species, Halolimnohelix langi, known from the Ituri forest) and the 
paucity of operculate land snails, the Cyclophoride being thus far known 
only by two species of Cyclophorus and one minute Cyathopoma. The 
relative abundance of the several groups of land snails is best expressed 
in figures: of the 150 forms known from the lowland forest east of 25° E., 
58 are Achatinide, 46 are Streptaxide, and 32 Zonitide, the remaining 
14 forms being divided between the Urocyclide (2), Vaginulide (4), 
Cyclophoride (38), Succineidee (2), Enidee (2), and Helicidee (1). 
The list of land mollusks obtained at Penge by the junior author in 
February, 1914, may be taken as a fair example of molluscan life in the 
virgin Congo forest. At the time of the writer’s visit this locality, 
situated on the upper Ituri, was practically untouched by man. A 
number of clearings were being made affording unusual opportunities 
for a study of plant and animal life in primeval conditions. The month 
of February that year was remarkably dry, hardly a shower being ob- 
served. Asa result the soil of the forest became gradually so dry that 
mollusks had to hide away for estivation. Perhaps this prolonged 
drought explains partly the scarcity of snails experienced by the writer, 
for hardly any of the species enumerated were found in large numbers, 
and in most cases only dead specimens could be obtained. The absence 
of Limicolaria, Achatina and Vaginulide in the collection must probably 
be explained in the same way. Moreover, mollusks of these three groups 
are rather rare in the virgin forest proper, being usually found in native 
plantations or on the grassy banks of streams, two kinds of habitats not 
to be found at Penge at the time of the writer’s visit. The best collecting — 
was done in scratching the superficial layer of humus and earth in well- 
shaded parts of the forest. In this connection it is necessary to mention 
that, notwithstanding the,luxuriance of the vegetation, there is but little 
decaying vegetable matter on the soil of the Congo rain forest, considerably 
less so than in temperate forests. The combined efforts of termites and 
fungi, together wilh the rapid decay by fermentation due to moisture 
and heat, seem to rapidly dispose of dead leaves, branches and tree- 
trunks. | 
