560 Bulletin American Museum of Natural History [Vol. LIII 
Total annual rise in meters 
Mean Maximum 
RG MOO Hi CaN VAL Gr eran tene cee bakes say Os oes gate ayo 3.65 5.35 
= a ctah 9 dS | fo So PNR Eee ey Chat Mra [ph ay OR ie A tt ee 3.45 4.40 
ae tr FOOD OLY ILO? . ces bree so oa 8 3.40 5.60 
Denar boa ee et es ee Ea 6.80 7.40 
€ SIE ULE aero) wastes Gla AAR Wea haat atel Sit Che 4.50 5.55 
ef oF EE WIGAN ROMEL Os. 2 i ovasy ros AS Gs kta GP alas Ae i Andee ae Cg 6.50 7.00 
aoa oe INOLaeie ai ae i eh Benue h ss Rten pute Menu i a iy 2.30 Say AS 
iS MONIC CRON Ee die tera Sire ate aie Ne cra sy teal tne isis 3.50 4.32 
The frequent changes of level not only modify the depth of the 
water and the strength of the current, two factors of great importance 
in the life of aquatic mollusks, but they also cause continual shifting 
about of the deposits of silt, thus rendering the existence of fresh-water 
mussels especially quite precarious. These many mechanical disturb- 
ances probably also account for the almost total absence in these shal- 
low flowing waters of aquatic plants which could offer shelter and food to 
aquatic snails. 
Just before entering the region of the Cataracts leading to its 
estuary, the Congo River suddenly broadens into the lake-like expansion 
of Stanley Pool. Being readily accessible from the coast and, as it were, 
the gateway to the interior, it has been visited by several collectors, so 
that its fauna is fairly well known. The Pool is some 24 kilometers long 
and 16 kilometers wide, obstructed by numerous sand banks and a 
number of islands. The large, alluvial island of Bomu, in the center, 
occupies much of its area. The surrounding country is hilly and along 
the right shore the banks are generally raised. At the extreme northern 
end they form bluffs of white sandstones, the so-called Dover Cliffs. - 
The left shore is usually lower and frequently covered with papyrus 
swamps. The following is a list of the species at present known from 
that region: . 
Physopsis africana Parreysia leopoldvillensis 
Pila leopoldvillensis Celatura elegans 
Lanistes congicus fraternus " xequatoria 
if bicarinatus re rotula 
a ovum var. major Aspatharia protchet 
Viviparus leopoldvillensis ‘i chapini 
Melanoides kinshassaensis Mutela hirundo 
a liebrechtsi TP CAeRey 
Physopsis, Pila, and Lanistes of this list are found mostly in the 
swamps which border the Pool. Melanoides and the several Unionidse 
and Mutelide live on and in the sand banks and mud bars. Some of these, 
such as the elegant Melanoides liebrechtsi, are quite abundant in spots. 
