1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 467 
basin, as for instance near Niangara. Cliffs and hills of limestone have 
also been reported along the western shores of Lake Tanganyika, and it 
is perhaps in such places that one should look for the Clausilia which 
was described many years ago from the southern end of that lake and 
has not. again been collected. In the narrow Atlantic coast strip of 
recent Mesozoic and early Tertiary strata, the molluscan fauna appears 
to be exceptionally rich wherever the soft, calcareous sandstones of 
these formations come at or near the surface. In such a location, at 
Zambi, Mr. H. Lang and the junior author collected many hundreds of 
specimens of land snails representing some eleven species. In the eastern 
Congo too, calcareous marls or sandstones of fresh-water origin and of 
very recent, Pleistocene or perhaps post-Pleistocene age, form much of 
the subsoil of the plain at the southern end of Lake Edward and extend 
into the hills farther south to beyond Rutshuru. These strata contain 
many fossil shells which are often well preserved and appear to belong to 
the same fluviatile species as live nowadays in Lake Edward. ‘In certain 
of the ravines which cut through these lacustrine deposits, abundant 
collections of land snails may occasionally be made. Thus, no less than 
twenty-four species of terrestrial mollusks were obtained by the junior 
author during about six weeks stay at Rutshuru. Finally, the extra- 
ordinary richness in molluscan life of the Kivu volcanoes is certainly 
partly due to the abundance of lime-containing minerals associated with 
weathered eruptive ashes and lavas. 
The topography of the country (Map 9) is not without its importance 
for the distribution of slow-moving, strictly terrestrial animals such as 
land snails and slugs. Where the country is level or but little and gently 
undulating, as is the case over easily two-thirds of the Belgian Congo, the 
environment is very uniform and there are few obstacles to dispersal 
over large areas. This factor, together with the similarity in climate and 
vegetation throughout the rain forest and savannas of the Congo low- 
lands, may explain the wide horizontal distribution of certain species. 
In higher and more rugged country, conditions are much more favorable 
for variation, isolation, and consequent intense speciation. This is quite 
conspicuous in the eastern highlands of the Belgian:‘Congo, where every 
mountain range or peak, every ravine may yield peculiar forms, while the 
several slopes, exposed to different climatic conditions, often exhibit 
conspicuous faunal peculiarities. As so very few of the mountains and 
valleys of the eastern highlands of the Congo have thus far been investi- 
gated, it is safe to prophesy that several hundreds of species of terrestrial 
‘mollusks will yet be discovered there. In the present state of faunistic 
