1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 521 
constitution of the soil, the concentration and nature of dissolved sub- 
stances, and the movements, depth, and volume of the water are among 
the chief conditions with which they have to contend. On the whole, 
aquatic mollusks appear to be more sensitive to relatively slight changes 
in their surroundings than most terrestrial species. On the other hand, 
many fresh-water snails and mussels are extremely plastic, readily ex- 
hibiting changes in their shape or behavior as a result of altered ecological 
conditions. | 
In tropical Africa the temperature of the water remains fairly con- 
stant,’ except in certain ice-fed small streams or ponds at very high 
altitudes; but not much is known as yet of the molluscan fauna of such 
cold waters. Mt. Ruwenzori, for example, possesses a number of glacier- 
fed streams and lakes, but the few aquatic mollusks known from the 
Range, viz., Lymnzxa natalensis, Planorbis bridouxianus, and Pisidium 
ruwenzoriense, were obtained in the lower regions or in the foothills. 
The Pisidium alone is recorded from the base of the Kichuchu rock- 
shelter, in about 3,000 m., and, as the genus is generally found, in Africa 
at least, in higher mountains, it may well represent a boreal element in 
the fauna. Some of the peculiarities of the aquatic molluscan fauna of 
Upper Katanga possibly are due also to the low nocturnal temperatures 
that prevail there during the dry season (see p. 518). 
The temperature of certain streams and ponds is considerably raised 
by the outflow of thermal springs. In North America and Europe certain 
Lymneide and Physide produce dwarfed forms in abnormally warm 
water. Thus, Lymnexa peregra var. thermalis Boubée, of the hot springs 
of the Pyrenees (in water at 45° C.) and Vosges, and L. p. var. gezsericola 
Beck, of the Iceland geysers (in water at 42.5° C.), are decidedly thinner 
and smaller than the typical form. Physa aurea Lea, apparently a 
dwarfed form of P. heterostropha Say, was described from Hot Spring, 
Bath Co., Virginia, living indiscriminately in water having a temperature 
of from 13° C. to 41° C. .In other cases the increased temperature does 
not seem to affect much the appearance of the shells. Raymond (1852, 
Journ. de Conchyl., II, p. 325) records finding Melanoides tuberculata in 
hot springs (32° C.) near Constantine, Algeria. However, but very few 
aquatic mollusks—some twenty species in all—are known with certainty 
to thrive at temperatures above 30° C.?: Neritina thermophila v. Martens 
(52° C.; New Britain); Bythinella thermarum Bourguignat (42° to 46° 
1In some of the larger lakes there is an appreciable change of temperature with increasing depth, as 
shall show for Lake Tanganyika. ; ents 
iy 2See Pelseneer’s recent oven of the mollusks of thermal waters. (1920, Mém. in 8° Ac. Belgique, Cl. 
des Sci., (2) V, pp. 499-506). 
