502 Bulletin American Museum of Natural History [Vol. LITT 
during June. These dry periods sharply separated two wet seasons of 
unequal duration, one with a maximum of 4.59 inches (116.5 mm.) in 
April, the other with 8.16 inches (208.2 mm.) in September. 
The variety of topographic conditions, with their concomitant 
changes in the nature of the soil, exposure, and climatic conditions, 
inevitably produce in this district many local differences in the nature of 
vegetation and animal life. The district is, therefore, of more than 
usual interest to the ecologist. Although savannas (Pls. LVI and 
LVII) are the dominating plant formation, yet in parts with a heavy 
rainfall and dry spells of relatively short duration, one finds rather ex- 
tensive patches of true rain forest of the West African lowland type. 
These forest islands are all situated in Uganda, the most important being 
those of Mabira, Buddu, Lwankoba, Kibale, Bugoma, and Budongo. 
With the exception of the Buddu Forest, they appear to be remnants of a 
former extension of the Guinean rain forest. Their vegetation shows un- 
mistakable West African affinities, and this is true also for their higher 
animal life, especially for mammals and birds, and for some of the insects 
(Lepidoptera, Hymenoptera). A comparative study of their molluscan 
fauna with that of the Ituri and Semliki rain forest would certainly be of 
much zodgeographic interest. 
The savannas which cover the district in Belgian territory belong to 
a number of types. Perhaps the most common is a short or medium- 
sized grass-land, chiefly composed of Themeda triandra Forskal and some 
of the smaller Andropogons, and almost destitute of woody vegetation. 
The grass rarely reaches 2 m. and is usually not over 1 m. high. This is 
quite typical of the gently undulating country near Irumu and south of 
Beni. In the exceedingly hot and low alluvial plain on the southern 
shore of Lake Edward, the grass becomes shorter still and grows in spaced 
tufts, so that the country has all the appearances of a steppe. Such dry 
savannas are practically without molluscan life, as the soil is often rocky 
and the animals can find no shelter against the scorching rays of the sun. 
The country is often deeply ravined by the heavy rains and it is quite 
indicative of the poverty of molluscan life that but very few dead shells 
can be gathered from the débris washed down into these gulleys. In the 
Semliki valley one also meets with bush savannas mostly of the thorny, 
subxerophytic type. On sandy, alluvial plains Acacia savanna, with 
umbrella-shaped trees, is quite frequent. Moreover these more wooded 
varieties of grass-land are hardly any richer in mollusks. On the lower 
slopes of the highlands, where rock débris washed down from the moun- 
tains form a fertile and rather moist substratum, there is usually a 
