1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 505 
and across Kenya Colony to Lake Nyasa and the lower Zambezi. At its 
northern end the trough-shaped basin of Lake Nyasa is joined by a 
branch from the northwest which encloses Lake Rikwa and connects in 
the neighborhood of Karema, through a bifureate breach in the eastern 
rim of Tanganyika, with the west-central African ALBERTINE Rut. 
This branch contains Lake Tanganyika and continues northward along 
the Ruzizi Valley, Lake Kivu, Lake Edward, the Semliki Valley, and 
Lake Albert to the Upper Nile. In many places the western scarp of this 
trench is extremely abrupt; but on the northern shore of Lake Kivu the 
floor and walls have been buried and partly obliterated by the volcanic 
activity of the Mfumbiro Range. In the Belgian Congo the Albertine 
Rift may be followed over a distance of nearly 1,200 kilometers; its 
width usually varies between 40 and 80 kilometers, though it 1s much 
narrower in the Semliki and Ruzizi Valleys. 
~ The several movements of the earth’s crust which have eventually 
led to the African rift valleys in their present shape were accompanied 
by uplifts of varying importance, giving rise to the East African highlands. 
Their distribution, altitude, and extent probably varied from one geologic 
period to another. Furthermore, there is ample evidence of a former, 
extensive glaciation on Mt. Kilimanjaro, Mt. Kenya, and Mt. Ruwen- 
zori. On Ruwenzori, to judge from the peculiar U-shaped higher valleys 
of the range, pointing to their formation by advancing glaciers, and from 
the presence of striated rocks and moraines at low levels, the glaciers at 
one time may have extended 10 to 12 kilometers beyond their present 
termination. On Mt. Kenya traces of former glaciation have been found 
some 1,500 m. below the actual glaciers. It is probable that this more 
extensive glaciation of the Central African mountains was contempora- 
neous with the Pleistocene glacial periods of Europe. At that time the 
climate of Central East Africa was not only much cooler over a large 
part of its area, but also considerably more moist. ‘l'emperate or moun- 
tain rain forest of the type found nowadays between 2,000 m. and 2,800 
m. in the Eastern Montane District, could then occur at much lower 
altitudes and may thus have covered a large part of eastern equatorial 
Africa, allowing a wide horizontal distribution of its peculiar floristic 
and faunal elements. Later, upon the retreat of the glaciers and the 
eradual desiccation of the country, the temperate forest disappeared in 
the lower highlands and persisted only as disconnected islands in 
the higher mountain ranges. In each of these patches isolation later 
permitted the production of peculiar species. In short, we believe that 
the various highland areas which nowadays make up the Hastern Mon- 
