1927] | Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo DLE 
toward the close of the rainy season. We have called attention before 
to the great zodgeographical significance of this feature. 
The course of the Nile is quite different from that of the Congo. 
Except at times of flood, when it inundates the surrounding valley, the 
river is comparatively narrow, its width varying between 300 m. and 
1,000 m. below Khartoum. It generally flows through open savanna 
country or desert, within well-defined, flat banks, deprived of woody 
vegetation, and it contains very few islands. The waters carry but 
little organic matter in suspension; in the lower reaches they are turbid, 
being loaded with muddy silt, especially when swollen by the rains of the 
Abyssinian highlands. The water of the White Nile and its tributaries is 
much clearer, since it descends from the clear lakes and is in addition 
filtered by the swamps of the Bahr-el-Ghazal. Near its confluence with 
the Bahr-el-Ghazal, the Albert Nile crosses a very extensive swamp, in 
the country of the Nuer, between 7° and 10° N. Here an area of some 
30,000 square miles forms an alluvial plain, mostly inundated through- 
out the year and covered with luxuriant aquatic plant growth, chiefly 
papyrus rushes (Cyperus Papyrus Linnzeus), ambatch trees (Herminiera 
Hlaphroxylon Guillemin and Perrottet), and certain floating grasses — 
(Vossia procera Wallich and Griffith, etc.). At the season of high water 
patches or even whole islands of this vegetation break loose and come 
down the current, forming the accumulations known as ‘‘sudd”’ (or 
“sadd’’), which block up the channels in the swampy part of the river. 
The malacological fauna of the sudd region appears to be quite similar to 
that of the swampy country near Lake Kisale, which we have studied in 
detail above (p. 543). It is likewise characterized by the abundance of 
Lanistes and Pila.! | 
The Lake Plateau basin furnishes by far the greatest volume of 
water to the Nile and comprises within the boundaries of the Belgian 
Congo a narrow, but quite important strip of territory. In the north- 
eastern borderland of our territory the Congo-Nile divide is constituted 
by a conspicuous ridge of highlands nowhere below 1,000 m. and fre- 
quently reaching 2,500 m. to 3,000 m. above sea level. It at first follows 
from north to south the western scarp of the Albertine Rift, within ten 
1For recent accounts of the mollusks of the Bahr-el-Ghazal see: oe: 
Sturany, R. 1913. ‘Liste der von Prof. Dr. F. Werner im Sommer 1904 im 4gyptischen Sudan und 
bei Gondokoro gesammelten Mollusken.’ Sitz. Ber. Ak. Wiss. Wien, Math. Naturw. Kl., CX XII, Abt. 
Bs . 549-556. 
a C.R. and Haas, F. 1913. ‘Ona collection of land and freshwater shells from the Upper 
Nile region.’ Proc. Malacol. Soc. London, X, pp. 355-361. 
1915. ‘Beitrage zur Molluskenfauna des Sudans.’ Zool. Jahrb. Abt. Syst., X XXVIII, pp. 371-384, 
Pl, exe, 
Longstaff, Jane. 1914. ‘Ona collection of non-marine Mollusca from the southern Sudan.’ Journ. 
Linn. Soc. London, Zool., XX XII, pp. 233-268, Pls. xv1i—xvui1. 
