Freshwater Mussels Dives 
receives most of it. Its temperature is therefore much higher than if 
the heat became distributed vertically, and higher than when the lake 
becomes holothermous in autumn. This results in a heightened metabolic 
rate on the part of the inhabitants of the epilimnion, while the abysmal 
bottom on the contrary is rendered unfit for the production of many 
living thing's. 
(b) Oxygen, Carbon Dioxide, and Carbonates. The water of the 
epilimnion receives most of the sun’s energy that is not reflected from 
the surface. Only here is photosynthesis effective, and here the phyto- 
plankton has evolved methods of flotation which keep the lake’s minute 
inhabitants mostly near the surface. For these reasons oxygen produc- 
tion is virtually limited to the epilimnion. Currents due to wind are set 
up which distribute the epilimnetic water from one part of a lake to 
another. The return currents pass underneath, next to the thermocline. 
For mussels the situation is perfectly adapted to secure well oxygenated 
water so long as they remain above the level of the thermocline. Yet 
even more striking than the oxygen curve of Winona Lake is the in- 
crease of the carbon dioxide. (Scott 716, p. 34.) 
Were a lake bottom in the hypolimnion entirely suited in other 
particulars to support mussel life, the conditions of temperature and 
oxygen would make it virtually uninhabitable. Sphaeriwm has been 
collected from bottom of various depths down to eighty feet, where, 
during summer, it exists under almost anaerobic conditions. 
The increase in acidity as we read downward in a lake means a 
corresponding reduction of the available carbonates, which is of im- 
portance in shell formation. Most of the marl deposition occurs in 
shallow water. The lime cycle is a function of the epilimnion almost 
wholly. 
In all respects we may say that the stratification of a lake tends 
toward increasing the habitability of the epilimnion at the expense of 
the hypolimnion. The turnover in autumn is rendered harmless to 
mussels through the thorough mixing, and through the temperature re- 
duction. 
(c) Food Supply, ete. The currents of the epilimnion are no less 
important to the Unionidae in that a constant renewal of the food sup- 
ply is effected. That it is entirely sufficient is shown by the fact that 
freshly collected mussels are never without plankton in the intestine, or 
without a crystalline style. (Allen, ’14, and ’21.) 
Evermann and Clark (’17) have stated as a foregone conclusion 
that rivers are the abode of mussels pan excellence. And it is true 
that there are more species and larger individuals. But I cannot 
wholly agree with their explanation. They say it is due to the changing 
water of the current, abundance of food and dissolved oxygen. Yet it is 
not explained why lake beaches are inferior in these respects to river 
shoals. In the former we have a slower, though no less steady, move- 
ment of water. The dissolved oxygen exists in great concentration, even 
to supersaturation. The plankton content of a lake surface is far in 
excess of that of most rivers. The average temperature of the lake 
habitats through the year is probably higher than in rivers, due to 
temperature discontinuity. Since these things are true, the metabolic 
