1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo BOT 
and local races throughout the Congo forest, they should be extensively 
collected, great care being taken to note which of the forms occur together 
and under what conditions they live. 
(2) In the several stretches of rapids and falls which interrupt at 
various points the course of the Congo and of practically all of its 
affluents, the stony substratum and the violence and large volume of 
flowing water naturally exclude most mollusks. These are, however, 
found in fair numbers in the more quiet parts, in the back-waters and 
pools between the rocks of the shore, as we have explained above. 
True falls, with leaps of several feet, are not very common in the Congo, 
except in the mountainous country of Upper Katanga. PI. LXIV, fig. 2, 
represents the falls of the Tshopo River near Stanleyville. The usual 
condition is that of rapids, such as shown in Pl. LXVI by a photograph 
taken on the Aruwimi River at Panga. Turbulent water spreads over 
a wide bed of boulders, which are completely submerged at high tide or 
protrude as islets with grasses or even shrubs at the season of low water. 
On the larger rivers these rapids can be passed almost everywhere by 
native canoes, at least at the proper season, although in rivers like the 
Aruwimi-Ituri and Uele-Ubangi, where they are extremely frequent, 
navigation is dangerous and rather exasperating. 
Rocks and stones fully exposed to the flow of the water are, asa rule, 
covered with a peculiar growth of mosses, liverworts and certain remark- 
able, algee-like dicotyledonous flowering plants belonging to the families 
Podostemonacee and Hydrostachyacez. Whether any minute mollusks 
are living within the dense floating cushions of these solidly anchored 
plants is not known. It is possible that some of the smaller species of 
Cleopatra, most of which are known from dead specimens only, should be 
looked for in such a location. The rocks themselves, however, are usually 
coated with the valves of the oyster-like Etheriide (Etheria elliptica in its 
various forms), one of the valves being solidly cemented to the stone by 
the whole or major part of its surface. The free surface of the shell is 
usually covered with fresh-water sponges (Spongillida),! while in the 
crevices of the somewhat spongy valves minute clams of the genus 
Hupera are rarely lacking. Cleopatra broecki Putzeys was found upon 
valves of Etheria in the Aruwimi River and Ancylide have been taken 
under similar conditions in Upper Katanga. 
1See Weltner, W. 1913. ‘Siisswasserschwimme (Spongillidze) der Deutschen Zentralafrika-Expedi- 
tion, 1907-1908.’ Wiss. Ergebn. D. Z. Afr. Exp. 1907-08, IV, pp. 475-485 (describes two species growing 
on shells of Htheria in the Aruwimi River). 
Annandale, N. 1913. ‘Notes on fresh-water sponges, XV. Sponges from shells of the genus 
| Mtheria.’ Rec. Indian Mus., IX, pp. 237-240. 
