524 Bulletin American Museum of Natural History [Vol. LIII 
as drop from fish in shifting sand and silt, die soon, being unable to fix 
themselves. It is quite possible that this explains the practical absence 
of Unionide over considerable stretches in the larger rivers of the Central 
Congo basin where gravel bars and stony situations are practically absent. 
The Unionide appear to be replaced there by the Mutelide. Untfor- 
tunately, the life-history of the African Unionide is still completely 
unknown. | 
The preferred habitat of each species is, of course, not due exclusively 
to the physical nature of the bottom, which moreover is itself the result 
of the movements, depth, and volume of the water. In shallow ponds or 
puddles and in muddy swamps the water is stagnant and there is usually 
abundance of aquatic vegetation so that much decaying matter is mixed 
with the mud, favoring the development of lower algee on which certain 
aquatic mollusks feed by preference. In the larger rivers the current is 
very slight near the banks so that thick deposits of alluvial matter are 
formed; mountain streams, on the other hand, have swift-flowing waters 
rapidly carrying away all alluvial matter except coarse gravel, and the 
mechanical force of the water reaches its maximum in the rapids and 
cataracts, which are an extremely characteristic feature of all the head- ‘ 
waters of the Congo basin.!. The mechanical action of the current not 
only regulates the distribution of the several species that inhabit a river 
system, but in some cases it may modify the morphological features of 
the shell.2 In the North American Jo fluviatilis the smoother forms occur 
in the upper reaches where the current is stronger. while the spinose 
varieties are found farther downstream in more quiet water.? Simroth 
has similarly sought to explain by differences in the rapidity of the current 
the occurrence in the rapids of the Congo of two forms of E. elliptica: 
the smooth form (typical ellzptica; mut. nidus hirundinis Simroth) being 
the one of more quiet situations; the spinose form (var. tubifera Sowerby; 
mut. tubulifera Simroth), the one of swift currents.4 Whether this is 
actually the case has been disputed, as the two forms have been found 
associated 
1According to T. J. Headlee (1906, Biolog. Bull., XI, pp. 305-318), the physical character of the 
pottom is the most potent factor in regulating the distribution of fresh-water mussels in small lakes. 
“See Jordan, H. 1881. ‘Einfluss des bewegten Wassers auf die Gestaltung der Muscheln aus der 
Familie Najades Lam.’ Biol. Centralbl., I, pp. 392-399. 
March, Margaret C. 1911. ‘Studies in the morphogenesis of certain Pelecypoda. (1) A preliminary 
note on variations in Unio pictorum, Unio tumidus and Anodonta cygnea.’ Mem. Proc. Manchester 
Litt. Phil, Soc. .b-V; 2; No.8; pp. 1-18, 1 Pl. 
Jel EE. L906. ‘Einfluss des bewegten Wassers auf die Gestaltung der Muscheln aus der Familie 
Unionide Flem.’ Nachrichtsbl. Deutsch. Malakoz. Ges., XX XVIII, pp. 38-44, 64-68. 
e ’Adams, C.C. 1900. ‘Variation in Jo.’ Proc. American Assoc. Adv. Sci., 49th Meet., pp. 208-225, 
S. I-XXVII. 
1915. ‘The variations and ecological distribution of the snails of the genus Jo.’ Mem. ion. 
Ac. Sci., Washington, D. C., XII, 184 pp., 61 Pls. i cae 
‘Simroth, H. 1980. ‘Ueber einige A\therien aus den Congo-fillen.’ Zool. Anzeiger, XIII, pp. 662- 
664 (also 1894, Abh. Senckenberg. Naturf. Ges., XVIII, 3, pp. 273-288, 1 Pl.). 
