490 Bulletin American Museum of Natural History [Vol. LIII 
in the basin of the Mongala River form one immense wooded swamp; 
while in the lowest part of the Congo bowl, south of the great bend of the 
stream, the basins of the Lulonga, Ikelemba, Tshuapa-Busira, and Mom- 
boyo-Luilaka Rivers are almost entirely under water part of the year. 
The woody vegetation of these periodically inundated areas is rather 
uniform, while the muddy soil itself is frequently almost bare. The in- 
undated forest is, however, the domain of creepers, lianas, and rattan 
palms, which frequently form almost impenetrable thickets. Ecological 
conditions of inundated woody areas are very unfavorable to land mol- 
lusks, as snails and slugs too often run the risk of being drowned. Yet 
we have observed that low, muddy places in the Congo forest seem to be 
the preferred haunts of the land operculate Cyclopherus intermedius E. v. 
Martens, which is frequently found on the decaying leaves covered with 
the black, slimy ooze left by the retiring flood. 
The virgin or primary rain forest on dry ground is of much greater 
interest to the biologist. It is typically developed in the higher and more 
hilly regions of the Aruwimi-Ituri basin (Pl. X LIX) and to the east of the 
Lualaba. The flora and fauna of these eastern forests are extremely 
varied, much more so than anywhere else in the West African Subregion.} 
This is true too for mollusks, for we note that of the four hundred-odd 
forms of terrestrial snails and slugs known at present from the Belgian 
Congo, about 150 have been found in the small area of lowland Rain 
Forest east of the 25th meridian. 
The molluscan forest fauna of the Congo basin is almost entirely 
made up of Achatinide, Streptaxidee, and Zonitideze. In the Achatinide 
the larger Achatinine are represented by a fair number of species of 
Achatina, Callistoplepa, Burtca, Limicolaria, and Perideriopsis; but the 
smaller Stenogyrine of the genera Homcrus, Ceras, Nothapalus, Subulina, 
Pseudoglessula, Pseudopeas, and Curvella are much more abundant in 
species. The rapacious snails of the family Streptaxide are one of the 
conspicuous elements of this fauna, presenting many beautifully shaped 
or elegantly sculptured forms of Hdentulina, Marconia, Gonaxis, Strep- 
tostele, Varicostele, Ptychotrema, and Gulella. The forest Zonitidee belong 
to two types: some have well-developed shells, more or less Heliz-like 
in shape (Thapsia, Ledoulxia, Trochozonites, and Kaliella); while in the 
Helixarionine, which are often found in profusion, the shell is degener 
1Mildbraed (1914, ‘Wiss. Ergebn. D. Z. Afr. Exp. (1907-08),’ II, 7, pp. 667-688) has given a satis- 
factory description of the extent and botanical characteristics of the Ituri Forest. His list of floristic 
elements could, however, at present be considerably extended. The many additional genera and species 
collected by the junior author during his journey of 1914-1915 still further bring out the great similarity 
of the eastern Congo forests with those of Gaboon and Cameroon and the floristic unity of the Lower 
Guinea Forest District. 
