. 1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 495 
500 m. and 1,000 m. above sea-level, and its topography is exceedingly 
uniform. It is a much worn-down peneplain of Archean and crystalline 
rocks, usually covered with a thick coat of lateritic surface soil. Occa- 
sionally the old, denuded mountain ranges have left witnesses in the 
shape of granite or gneiss hills (so-called ‘“‘Inselberge” or monadnocks) 
which are quite distinctive of the Congo-Nile divide (Pl. LIV, fig. 1). 
These granite hills are often broken up by crevices in which moisture 
accumulates, so that dense thickets of entangled vegetation surround the 
base, affording a welcome shelter for many animals. Such a habitat 
might, therefore, be well worth investigating for mollusks, especially in 
the dry season for estivating species. — 
The climate of the northern Congo savanna is characterized by a 
sharp division of the year into a dry and wet season. During the summer, 
from April-May to September-October, it rains profusely and animal and 
plant life are then at their height. The balance of the year, however, is 
practically rainless and toward January the drought is so pronounced 
that plant life is almost at a standstill and many animals have either 
migrated to other parts of Africa or have hidden away for estivation. 
The country is usually covered with grass-lands of the various types that 
occur elsewhere in the West African and Guinean lowlands and which 
may best be included under the general designation of savannas (Pls. 
LII and LIII). MHigh-grass savanna (with Pennisetum, Panicum 
maximum Jacquin, many Andropogons, etc.), practically without 
woody vegetation, appears to cover much of the country south of the 
Ubangi; while farther east, in the basin of the Uele, bushes and low trees 
(Anona senegalensis Persoon, Acacia, Strychnos, Hymenocardia, Gardenia, 
etc.) are scattered about more abundantly. On the border of the Anglo- 
Egyptian Sudan one even meets with savanna forest, though the trees 
never attain a great height or diameter. Here, too, some Sudanese 
elements appear in the flora, perhaps the most interesting of them being 
Protea madiensis Oliver. The district contains also many forest galleries 
(P]. LIII) which extend along the rivers from the equatorial belt of 
rain forest and gradually narrow as one travels northward. In these 
galleries one finds, but in a somewhat impoverished condition, the flora 
and fauna of the rain forest proper. | 
To judge from the foregoing ecological description of the Ubangi 
District, the malacological fauna is not likely to be abundant nor varied 
in species. Unfortunately, very little collecting has been done thus far 
in that area. Schweinfurth obtained a number of species on the Congo- 
