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1927] Pilsbry-Bequaert; The Aquatic Mollusks of the Belgian Congo 523 
conditions may then prevail in a country which a few months before was 
practically flooded. The mollusks left stranded by the drought eventu- 
ally die, unless they be able to bury themselves deep enough in the mud 
or to withstand prolonged desiccation by tightly closing the operculum. 
We shall have occasion to point out some interesting adaptations to 
periods of drought and temporary aérial life, in describing the molluscan 
life in the swampy plains of the Upper Lualaba. 
The physical nature of the bottom of the water is of considerable 
importance in the life of aquatic snails and mussels. Thus, in the Belgian 
Congo it may be observed that the species of Lymnea, Planorbis, and 
Physopsis, prefer shallow ponds in the savanna country or even pools of 
water that accumulate in depressions of rocks and are partly filled with 
aquatic plants, so that the muddy bottom is rich in decaying vegetable 
matter. Lanistes and Pila are particularly abundant on the oozy mud of 
papyrus swamps. Many of the smaller Melanoides frequent the spits of 
shifting sand in the larger rivers, where they usually bury themselves a 
little below the surface of the sand. On the other hand, the bars of muddy 
silt near the quiet shores of shallow rivers are the peculiar habitat of most 
Congo Mutelide and Unionide. The Etheriide prefer a stony substra- 
tum on which to encrust themselves in oyster-fashion, and consequently 
are met with in rapids and falls.2. In a similar habitat many species 
of Potadoma and certain Bulimus keep in the crevices of the rocky ledges. 
Finally, the clear, torrential streams of more rugged, mountainous 
country nourish the peculiar smaller Amnicolide, such as Lobogenes of 
Upper Katanga, and Spheerlide, which live among the fine pebbles of 
the gravelly bottom. | 
An interesting case is offered by certain Unionide for which the 
presence of coarse gravel bars under swift water, at moderate depth, 
appears essential for the early development of the mussel. In North 
America Isely? has shown that in several species the parasitic stage of 
the glochidium is followed by an early juvenile condition, when the small 
mussel is attached by a byssal thread to a pebble of immersed gravel 
beds, under a fairly swift current. He believes that such juvenile mussels 
1The most remarkable adaptation of a fluviatile snail to periodically arid conditions is that exhibited 
by the South American “ Ancylus’’ moricandi d’Orbigny which in the Bolivian Chaco covers the open- 
ing of the shell almost completely with a calcareous, convex septum, apparently part of the shell and 
leaving but a small opening at one end. See Nordenskidld, E. 1903. ‘Ueber die Trockenzeitanpassung 
eines Ancylus von Siidamerika.’ Zool. Anzeiger, X XVI, pp. 590-593. Also H. A. Pilsbry, 1914, Proc. 
Ac. Nat. Sci. Philadelphia, LXV, (1913), pp. 668-671. 
2The much distorted shape of the valves of Etheria appears to us due rather to the irregular shape of 
their substratum, and not to the mechanical action of swift currents as Simroth (1890 ,Zool. Anzeiger, 
XIII, p. 662) suggests. ; gatie SO st 
vB. oe TO1t. ‘Preliminary note on the ecology of the early juvenile life of the Unionide. 
Biolog. Bull., XX, pp. 77-80. 
