494 Bulletin American Museum of Natural History [Vol. LITT 
at Nouvelle Anvers, very large numbers of Achatina sylvatica were seen 
by the junior author in October, 1910, and Lang and Chapin found that 
species abundant under similar conditions at Mobeka and Stanleyville. 
The foregoing description of ecological conditions and molluscan 
life in the rain forest applies to the forests of the Central Basin of the 
Congo. We have mentioned before that the forests of Gaboon continue 
southward into the Mayombe district to about 50 kilometers north of 
Boma. Though the botanical features of this ‘‘Lower Congo forest” | 
are essentially the same as those of the Upper Congo, nevertheless it is 
possible to point out certain differences which are probably due to 
climatic peculiarities. At Ganda Sundi, for instance, in 4° 55’ 8. and 13° 
5’ E., observations made during 1910-1913 gave for these four years an 
average annual rainfall of 54.81 inches (1,394 mm.), the wettest months 
being February with 9.31 inches (236.5 mm.) and November with 8.64 
inches (219.5 mm.); the driest months, June and July, were practically 
rainless. The difference of mean temperature between the hottest and the 
coldest month in 1913 was 6° .8 C. (February: 27°.9 C.; July: 21° .1 C.). 
The climate is, therefore, much more like that of the surrounding savanna 
country (see Southern Congo Savanna District) and the presence of rain 
forest in this region is merely due to the hilly and deeply ravined topog- 
raphy, and to the proximity of the sea, so that even in the dry season 
there is abundant moisture in the air and dense fogs are frequent in the 
morning. 
Moreover, the rain forest is rapidly disappearing in the Belgian 
part of the Mayombe under the combined efforts of native agriculture 
and of Kuropean tropical cultures. In recent years large parts of forest 
have been transformed into plantations of cocoa. We have been unable 
to find any published records of terrestrial mollusks for the Belgian 
Mayombe; yet it would be of great interest to know to what extent this 
fauna differs from that of the Upper Congo forest. It is quite possible 
that some of the peculiar forms of the Cameroon and Gaboon, such as 
Pseudoirochus, are to be found there. 
2. Ubangi Savanna District 
North of the continuous belt of Lower Guinea rain forest, a fairly 
flat and moderately elevated peneplain extends between the bend of the 
Ubangi and the Albert Nile, from1&° to 31° E. Similar country is found 
north of the borders of the Belgian Congo, to slightly beyond the Congo- 
Nile divide, which is here so little marked as hardly to be a noticeable 
feature of the landscape. This whole northern district lies between 
