518 Bulletin American Museum of Natural History [Vol. LIII 
At Elisabethville, for instance, in 11° 40’ S. and 27° 25’ E., at an altitude 
of 1,237 m., the temperature about the middle of the dry season does not 
exceed 29° C. at noon, while during the night it may drop to 2° C. The 
hottest months are September and October, that is just before and during 
the first rains, when a shaded thermometer registers from 35° C. to 36° C. 
toward noon and from 8° .2 C. to 13° C. during the night. The total 
annual rainfall for two years in that locality was respectively 42.7 inches 
(1,084 mm.) and 53.5 inches (1,358 mm.). In the shallow, open valleys 
of Katanga, known as ‘‘dembos,”’ there is often frost at night during the 
dry season and on the higher plateaus the thermometer may then mark 
as low as —3° C. or —4° C. just before sunrise. It thus appears that in 
many respects the climate of Upper Katanga places this region at the 
extreme limit of the tropical zone. In this connection 1t should be noted 
that extremes of temperature, especially minima, are of far greater im- 
portance for the life and distribution of animals and plants than the daily 
or seasonal averages usually given in meteorological tables. There can 
be little doubt that the occasional cold nights of the dry season are a 
foremost factor in preventing the spread of West African forms of life 
into Upper Katanga and that they are chiefly responsible for the sharp 
dividing line between the West African Subregion and the East and South 
African Subregion in the southeastern Congo basin. 
Owing to the peculiar combination of a wet season with abundant 
rainfall and a dry season with extreme arid conditions, the vegetation of 
Katanga is almost everywhere a savanna forest (Pl. LXIJI) with decided 
tropophytic characteristics and of which papilionaceous trees, especially 
of the genus Brachystegia, are the dominating element. Shortly after 
the end of the rains many of the trees drop their leaves; the grass remains 
short'and many of the perennials and suffrutescent herbs possess under- 
ground rhizomes or bulbs in which they store food and water for the 
periods of drought. This enables them to flower shortly before the first 
rains and usually before producing leaves. The long period of drought 
forces the land mollusks to estivate. Some, as Rachis braunst, merely 
remain on the trunk of savanna trees and secrete a substance with glues 
the margin of the aperture fast to the bark, while they daub the surface 
of the shell with dirt so as to be concealed. Others burrow in loose 
soil or seek shelter in the galleries of the huge termitaria which are 
extremely numerous over much of this district. This is the common 
method of estivation used by the Limicolarie and Achatine, which in 
addition close the aperture with a solid, caleareous epiphragm. The 
destructive action of the grass fires is not so much to be feared here by 
