526 Bulletin American Museum of Natural History (Vol. LIII 
The nature and amount of various solid substances in suspension, 
such as slit and humic matter, and of the chemicals in solution are fore- 
most among the environmental factors acting upon aquatic mollusks.’ 
In the first place, many of these animals breathe through gills and even 
in the case of the pulmonates, cutaneous breathing is often quite active, 
so that the concentration of oxygen in the water is particularly important 
for survival and development.? It has been shown that the effect of 
various concentrations of oxygen is modified by the hydrogen ions present; 
particularly the ill effect of a low amount of oxygen is increased by the 
high hydrogen ion concentration that accompanies it in water of low 
alkalinity.2 We have commented above upon the general deficiency of 
lime in the soils of the Belgian Congo and the consequent ill effects upon 
terrestrial mollusk life. The superficial waters likewise show a very low 
concentration of lime. Spence notes that ‘‘the ordinary river water (of 
the Congo) only contains 4 to 5 grains of solids per gallon and is almost 
like distilled water, except for the brown color of organic matter.’’? This 
scarcity of lime probably explains why mollusks are represented by so 
very few species and individuals over large stretches of the rivers, not- 
ably in the low, forested part near the equator, between the mouth of 
the Aruwimi and that of the Ubangi. Here the lack of lime in solution is 
accompanied by extreme acidity of the water due to an abundance of 
humic and decaying vegetable matter carried in colloidal suspension. 
That, in contrast, some of the great lakes offer a profusion of snails and 
mussels may be explained by a higher concentration of lime compounds 
due to active evaporation without much dilution by outflow... From a 
very careful experimental study of the effects of environment upon the 
growth of the North American Lymnzxa columella Say, H. S. Colton 
concludes that calcium salts in the water, particularly calcium sulphate, 
seem to be on the whole beneficial to the growth of the shell.6 More- 
over, the abundance or scarcity of lime in solution affects the various 
species of fluviatile mollusks quite differently, as justly pointed out by 
1K. Dupont |1891, Bull: Ac.’ Sei. Belgique, (83) XX, (1890), pp. 559-566] believes that the poverty 
of the Congo basin in fluviatile mollusks is due in the first place to the abundance of silt carried in sus- 
pension which, upon entering the organs of the animals, has a deleterious effect upon their functions. 
*See Willem, V. 1896. ‘Observations sur,la respiration cutanée des Limnées et sur son influence 
sur leur croissance.’ Bull. Ac. Sci. Belgique, (8) XXXII, pp. 563-577. 
$See Shelford, V. E. 1923. ‘The determination of hydrogen ion concentration in connection with 
fresh-water biological studies.’ Bull. Illinois Nat. Hist. Survey, XIV, Art. 9, pp. 379-395. 
£1923, Journ. of Conchology, XVII, p. 19. 
_ Many examples of the direct effects of varying concentrations of lime upon fluviatile mollusks are 
given by Pelseneer, 1920, Mém. in-8° Ac. Sci. Belgique, (2) V, pp. 573-575. S.G. Rich (1915, Science, 
N.S: XLII, pp. 579-580) has described Unio complanatus Dillwyn var. mainensis, with horny and some- 
what flexible valves covered with a very thick epidermis, from a pond on granite gneisses in western 
Maine, were ie Bae ae totally destitute of lime. 
olton, H.S. . ‘Some effects of environment on the growth of Lymnza columell % . 
Ac. Nat. Sci. Philadelphia, pp. 440-448. i a ae 
