1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 527 | 
J. W. Taylor. Some European species, he notes, “‘as Unio margaritifer 
and Neritina flunatilis, appear able to extract the necessary lime 
carbonate to form thick and heavy shells even from the waters of granitic 
districts, whilst other species, as Ancylus fluviatilis, under similar condi- 
tions seem unable to do so, their shells being unusually delicate and thin.’”! 
Most aquatic mollusks respond readily to relatively slight changes 
in the concentration of mineral matter, especially sodium chloride, affect- 
ing the osmotic pressure of their tissues. Some species, such as the Lym- 
nxa, Planorbis, and Bulinine, can apparently not stand in nature even a 
comparatively slight and gradual increase in salinity and consequently - 
they are but seldom found near the mouths of the rivers. Others, how- 
ever, Show a much greater plasticity: thus, at the mouth of the Congo 
one finds several species of Lanistes under highly brackish conditions, 
sometimes associated with typically marine forms; yet they have un- 
doubtedly been derived from species inhabiting the fresh waters of the 
interior of the Congo basin.? In this connection it is of interest that the 
junior author has found Lanzstes nsendweensis® in a decidedly brackish- 
tasting stream near Kiabwa, about 75 kilometers south of Ankoro., The 
reverse process of marine species becoming gradually used to brackish 
conditions is observed at the mouth of the Congo and of the other African 
rivers. In the interior of the African continent sodium and potassium 
chloride are usually scarce and generally absent from flowing and stag- 
nant water. In some of the larger lakes where evaporation is very rapid 
and little or no outflow takes place, salt accumulates and their water is 
more or less brackish. Lake Kivu is peculiar in the very high concentra- 
tion of magnesium salts. : 
It is unfortunate tha! analyses of the water of the Congo have been 
but rarely made. The data obtained in the Nile are nol applicable to the 
Congo system, since none of the head-waters of the Nile* run through 
swampy, rain forest areas such as occupy the equatorial parts of the 
Belgian Congo. It is much more likely that the water of the Congo may 
11894~1900. ‘Monograph of the land and freshwater Mollusca of the British Isles. Structural and 
general volume.’ (Leeds), p. 79. Wy 
2Eixamples of fresh-water mollusks that have become adapted to more saline conditions are given 
by Pelseneer, 1920, Mém. in-8° Ac. Sci. Belgique, (2) V, pp. 506-507 and 570-571. Melanoides tuber- 
culata occurs in highly brackish lakes of northern Africa, where it sometimes lives with Cardium edule. 
3Recorded as L. bourguignoni by Dautzenberg and Germain (see p. 188). 
40. Chadwick and B. Blount found that the water of Lake Victoria contains 0.135% of mineral 
matter (25.13% of which was Na, 6.96% Ca, 7.61% SiOz, and 1.92% Fe2O3). For comparison, we may 
quote the mean of 34 analyses of the water of the Miami River at Dayton, Ohio (the salinity of which is 
about mid-way between the extremes for waters of the Ohio River system): total salinity: 0.279%, 
11.04% of which is Na, 16.10% Ca, and 5.89% SiOz. These figures and those given for the Amazon in 
the text are taken from F. W. Clarke, 1920. ‘The data of geochemistry. U.S. Geol. Surv. Bull. 695, 
pp. 79, 91, and 105. The same author states (p. 106) that in the water of the Nile 10.36% of the dis- 
solved solids is organic matter, while this figure increases to 15.08% in the Amazon, to 24.16% in the Rio 
Tapajos, and to 59.90% in the Uruguay River. 
