1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 525 
In most of the larger lakes the extent and volume of the water, freely 
exposed to the action of the wind, result in the production of formal 
waves that break against the rocky shore or roll in surf-fashion over the 
beaches. Only types of mollusks that can solidly adhere to the rocks or 
possess other devices to withstand successfully the assaults of the waves, 
can thrive in such an environment and this factor has undoubtedly had 
its share in shaping the individuality of the lake faune. Even in smaller 
bodies of water the volume and depth appear to influence the size as well 
as the number of species of mollusks. It has been shown experimentally 
that the size of the shell in Lymnea stagnalis varies directly in proportion 
to the volume of the water in which it lives... Whether this is a general 
rule in nature appears nevertheless somewhat dubious, though in many 
cases the largest specimens of certain species were obtained from fair- 
sized rivers or ponds. In North America, Ortmann concludes from his 
study of correlation of shape and station in fresh-water mussels: 
“Certain Naiades change their shape along the course of one and the same 
river in such a way that, (1) the more obese (swollen) form is found farther down 
in the large rivers, and passes gradually, in the upstream direction, into a less obese 
(compressed) form in the headwaters; (2) with the decrease in obesity often an increase 
in size (length) is correlated; (3) a few shells which have, in the larger rivers, a 
peculiar sculpture of large tubercles, lose these tubercles in the headwaters. The 
question arises: what is the meaning of these changes in shape? No positive conclu- 
sion is as yet possible, chiefly for two reasons: first, that there are only some species 
(and comparatively few), in which this law is observed, while others positively do not 
show it; and in the second place, that, although the size of the stream undoubtedly 
is connected with this phenomenon, we do not know whether size alone is the essen- 
tial factor, or whether additional factors belonging to those constituting the small- 
stream community are responsible.’”2 
Grier found that Unionide from Lake Erie were as a rule smaller, had 
broader growth-lines, brighter colors and more polished epidermis, and 
were thinner than the same species or closely allied forms from the drain- 
age of the upper Ohio.® 
It has also been claimed that the number of genera and species of 
mollusks in each of the great lakes of Central Africa is directly propor- 
tional to the size of the lake. This interesting question will be taken up later. 
— Willem, V. 1896. ‘Observations sur la respiration cutanée des Limnées et son influence sur leur 
croissance.’ Bull. Ac. Sci. Belgique, (8) XXXII, pp. 563-577. Willem’s experiments seem to indicate 
that neither volume nor exposed surface of the water are the important factors regulating growth, but 
the amount of oxygen present in the water. See also F. C. Baker, 1911, Chicago Ac. Sci., Special Publ. 
No. 3, pp. 47-51. aR 
DAE _Ortmann. 1920. ‘Correlation of shape and station in fresh-water mussels (Naiades).’ Proc. 
American Philos. Soc., LIX, pp. 269-312, map. ; 
3See a series of papers on this subject by N. M. Grier: 1920, Ann. Carnegie Mus., XIII, pp. 145-182, 
Pls. 1-111; 1920, American Midland Naturalist, VI, pp. 211-243, Pls. 1-111, and pp. 247-285; 1922, op, 
cit., VIII, pp. 129-148. Also G. H. Ball. 1922, ‘Variation in fresh-water mussels.’ Ecology, III, pp. 
93-121. These publications contain extensive bibliographies. Further references are given by Pel- 
seneer, 1920, Mém. in-8° Ac. Sci. Belgique, (2) V, pp. 552-557. 
