1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 561 
(4) The physical conditions of the Congo estuary between Matadi 
and the Atlantic were briefly described above. In the upper reach, 
between Matadi and Boma, the deep river flows in one rather narrow 
channel, margined by rocky shores. The water is not in the least 
brackish and the current rather strong, often forming whirlpools around 
the more or less submerged rocky ledges of the banks. Mud flats and 
sand spits are almost totally lacking, so that ecologi¢al conditions are 
quite different from those found elsewhere in the Congo basin. Un- 
fortunately, but for the occurrence of the peculiar snail Pseudogibbula 
dupontt on immersed rocks at Vivi, the malacological fauna ‘of this 
reach is still an untrodden field. 
Owing to the fjord-like structure of the Congo estuary where the 
fresh-water only forms a superficial sheet over the deeper filling of sea- 
' water,! velocity due to slope alone is slight. Thus, the mean level of 
the stream at Matadi, some 130 kilometers from the sea, is but 0 m.1 
above mean sea level. It follows that the action of the tides is of con- 
siderable ecological importance. At Boma, about 80 kilometers from the 
mouth, this is as yet of little import, since the difference between high 
and low tide is at most 2 to 3 cm., at full and new moon during the dry 
season, and the water is not perceptibly brackish. Below this point the 
influence of the tides steadily increases. At Malela, 30 kilometers from 
the sea, the amplitude of the tide is from 0.1 m. to 1 m., according to 
the phases of the moon and the seasonal variations of water level. 
The malacological fauna undergoes quite decided changes as one 
progresses downstream.2 The few mollusks known from the Congo 
River at Boma are all typical fresh-water species: Lanistes intortus, 
L. adansoni, Czlatura stagnorum, and C.. stagnorum bome. 
The fauna of Zambi is more abundant, since that locality shows a 
variety of conditions: swamps with papyrus, rocky shores at the cliff-like 
banks below the mouth of the Lukunga River, and sand spits and mud 
flats in the shallow portions of the northern branch of the Congo (Pls. 
LXXI, fig. anda: 
Neritina oweniana Parreysia leopoldvillensts 
Lanistes tntortus Celatura elegans 
ds adansont *: stagnorum 
Melanoides langi zambiensis Mutela langr 
!The fauna of the deeper portions of the estuarine waters of the Congo is totally unknown. All 
mollusks obtained in this region are shore or littoral species living between the limits of the upper and 
lower tides and a few feet below the lower limit. ; 
2No representatives of the genera Lymnexa, Bulinus, Planorbis, Pila, Potadoma, and Cleopatra are 
‘known from the estuary of the Congo. Although it would be difficult to account for their absence there, 
at least in the region between Matadi and Zambi, yet it is hardly possible,that they should have been 
overlooked by all collectors. The discussion of ecological conditions in this region may best be followed 
by referring to the detailed map published in the Report of Land Mollusks (1919, Bull. American Mus. 
Nat. Hist., XL, p. 15). 
