1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 541 
country in shallow depressions which may completely dry up after the 
rains, when the mollusks either bury deep in the mud or survive only in 
the larger, permanent ponds. Such collections of stagnant water are 
usually abundantly provided with aquatic vegetation, especially green, 
thread-like confervoid alge. In the peneplain of the northern Uele 
district, at the divide of the Congo and Nile basins, water often stagnates 
in the depressions or crevices of granitic outcrops, especially of the 
flattened type known as ‘“whaleback.” Such a pool on granitic rock is 
shown in Plate LXII, figure 2, from a photograph taken near Yakuluku, 
close to the Congo-Nile divide, in October, 1911. At that time of the 
year, toward the close of the rainy season, water is in abundance and 
consequently the vegetation is quite luxuriant. During the dry season, 
however, most of these ponds gradually disappear, although water is to 
be found all year round in some of the deeper crevices. J. W. Taylor! 
remarks that the shells of the Lymneex “inhabiting tropical and sub- 
tropical countries are said to be usually much more constant and uniform 
in sculpture, as well as in size and shape, than their congeners from more 
northern districts; and their texture is also finer and smoother on the 
whole than that of species living in the colder regions.’’ This statement 
is well borne out by an examination of Congo specimens. The generally 
thin, fragile, and translucid texture of the African Lymnxz is evidently 
due to the scarcity of lime in the water of these granitic pools. Such 
small collections of stagnant water contain no operculate snails, but one 
should look there for minute Pelecypoda, such as Spheriuwm and Piszdium. 
Near the edge of the rapids and falls, which, as we shall see later, 
frequently interrupt certain stretches of the rivers, there are usually 
between the rocks small pools of stagnant water, renewed at intervals 
by the rise of the stream. Such pools probably offer the greatest variety 
of aquatic snail life of any habitat within our territory (except the 
shores of certain lakes). Plate LXIV, figure 1, gives the aspect of the 
rocky shore of the Congo River, at Stanley Falls, about a mile above 
Stanleyville, in August, 1909, that is at the season of lowest water. The 
following species were all obtained in the water puddles of that 
locality, and many of them were extremely abundant. 
Physopsis africana var. Melanoides kisangant congo 
Lanistes nsendweensis Potadoma ignobilis 
i procerus langi ay ponthiervillensis 
Ni grauerr | re a mut. spoliata 
Pila congoensis Cleopatra lang 
‘« microglypta A cara 
Melanoides wagenia Corbicula radiata 
yh ante egies idee Sich AN uh pak ee ee ere 
11894-1900, ‘Monograph Land Freshw. Moll. Brit. Isl. Struct. Gen. Vol.,’ (Leeds), p. 71. 
