1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Ces 545 
Another interesting habitat of this region are the channels that 
connect the lagoon lakes with the main stream of the Lualaba. Their 
fauna is somewhat different from that of the surrounding papyrus 
swamps, for one finds here also Unionidee and Mutelide. In the channel 
which serves as effluent to Lake Kabamba, near the village of Mulongo, 
the junior author obtained at the end of the dry season (October, 1911) 
the following species: Pila sp. (ovata?), Lanistes procerus langi, Vivi-~ 
parus unicolor, Celatura xquatoria, Aspatharia wissmanni, Mutela — 
rostrata, and M. iris. The mussels were all buried in the sandy bottom 
near the shallow banks. 
Mollusk Fauna of Flowing Water 
In many parts of the world the malacological faune of fresh 
waters possess a certain degree of individuality. This remark applies 
not only to the main hydrographic basins, but in many cases it is found 
that certain tributaries possess their own species or that certain species 
are peculiar to the upper or lower reaches of a stream while they are 
totally unknown elsewhere. It would even appear that in some cases 
large and deep rivers, especially such with muddy waters, act as barriers 
preventing the interchange of species between their tributaries. What 
is more, the same species may vary in size and shape in the several tribu- 
taries of a river or even in different parts of the same stream. These 
modifications were, of course, due to the effect of environment, as we have 
explained in our introductory remarks on the ecological factors which 
influence molluscan life, the resulting changes becoming permanent owing 
to prolonged isolation. 
Faunistic differences between the several river basins are, however, 
but little apparent in Africa. In the flowing waters many genera and 
even certain species extend over immense areas, regardless of water- 
sheds. Thisis especially true for Lymnza, Planorbis, Bulinus, Physopsis, 
and some of the Mutelidsz and Unionids. Many described species in 
_ these groups are hardly separable or at any rate show extremely close 
affinities. In the case of the Etheriide, for instance, all recent authors 
are fairly agreed that Africa possesses but one protean species, the 
profound differences in shape being possibly due to direct action of 
the environment upon the individual and also in part to age. 
The fresh-water molluscan fauna of Africa is far more homogene- 
ous than that of the land, and the faunal subdivisions of the Ethiopian 
Region which we have discussed above (p. 479) have hardly any signif- 
13ee Goodrich, C. 1921. ‘River barriers to aquatic animals.’ The Nautilus, XXXV, pp. 1-4. 
