BOOS es Bulletin American Museum of Natural History [Vol. LIIT 
Of the above list only Pseudoglessula boivinit and the thin-shelled 
Achatina glaucina appear to be Hast African elements, but even the 
Pseudoglessula is not quite typically so, since the junior author has also 
collected it at Malema in the central Congo forest. With the exception 
of Achatina glaucina and Rachis, which are typical savanna snails, the 
mollusks of Bukama were obtained in the moist and densely shaded oil 
palm groves that fringe the Lualaba in that locality. 
Some forty kilometers south or east of Bukama the West African 
plants and animals gradually disappear. As altitude steadily increases, 
they are replaced by genera and species of East and South African affinity, 
until one finally enters the Katanga proper, part of our Rhodesian High- 
land District. — 
4. .Uganda-Unyoro Savanna District 
This district comprises all the areas of our territory below the 1,500 
m. contour line in the region between the eastern limit of the Congo 
Rain Forest and the boundary of Uganda. It extends, however, con- 
siderably farther east to the northwestern shore of Lake Victoria and to 
the Victoria Nile. It forms, as it were, a West African wedge driven into 
the East and South African subregion and is on that account of particular 
interest, since many West African forms of life here reach their extreme 
eastern limits. In the northwest this district borders upon the Ubangi 
Savanna District. Although the transition between the two occurs in 
savanna country, yet there is an appreciable and rather rapid change in 
the ecological features of flora and fauna, as one proceeds from the 
savanna forest of the Congo Nile divide to the open grass-lands of Irumu 
and the lower Semliki valley. 
The subsoil consists almost everywhere of granitic and old crystalline 
or metamorphic, Palzeozoic rocks. These are usually weathered to a 
considerable depth into superficial layers of loam or laterite. In the 
valleys and on the shore of the lakes they are hidden beneath alluvial 
deposits of quite recent date, sand, gravel, grit, and clay, sometimes 
extremely rich in dead shells. In spite of this rather great uniformity 
of geologic structure, the country offers more varied ecological con- 
ditions than is generally the rule in the Belgian Congo. This is due 
primarily to the presence in this area of the deep, trough-like depression 
which forms the northern end of the Albertine Rift. The resulting topo- 
graphic disturbances are nowadays evident as large lakes, filling the deeper 
parts of the depression, important mountain ranges separated by deep, 
trench-like valleys, and high plateaux bordered by abrupt scarps. The 
