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132 THE AMERICAN MIDLAND NATURALIST 
contrasting columns, when the relation of the lake environment to 
this problem will eventually become clearer. 
LAKE ERIE 
Water colder than in Upper 
Ohio, with more even regulation of 
temperature. Currents: much less 
rapid than in streams, water less 
agitated except by moderate cur- 
rents carrying but little sediment. 
Bottom composed of pebbles, sand, 
or a mixture of these depending on 
region of lake, with coarser sedi- 
ment derived from wear of land. 
Temperature conditions favor more 
uniform production of food, while 
the water contains more lime. 
UPPER OHIO DRAINAGE 
Water- warmer than in L. Erie, 
greater extremes of temperature. 
Streams more rapid than currents 
of L. Erie and more greatly agi- 
tated, frequent falls and rapids, 
short stretches of quiet pools. 
Rivers carry a load of debris, mov- 
ing quickly over the bottom, con- 
sisting of mud, glacial till, cob- 
bles. Food conditions, (due to ex- 
tremes of temperature), are less 
stable, even if at times, food is 
more abundant. 
It may be noted here that Lake Erie varieties of shells as a 
whole possess brighter and clearer colors than their parent shells_of 
the Upper Ohio and Upper Mississippi; are exceptionally polished, 
and otherwise characterized in distinction by their well developed 
lines.of growth. It has also been suggested that certain depau- 
perate qualities of these shells, such as their dwarfed condition and 
thinness, may be due to the chemical quality of the water, (5), the 
influence of brackish water upon fluviatile species being well known. 
ITV.—Mernop. 
A common method of estimating the age of a mussel has been to 
count the so-called rings or lines of growth. Studies by previously 
mentioned investigators, (10, 2), have shown that, under certain 
conditions such a procedure would yield an unreliable estimate of 
the animal’s age. For example, mussels living in rivers undoubtedly 
form a growth ring every winter, but what was not certainly known, 
was that a period of summer drought, or even temporary removal from 
the water, would cause a superficially similar reaction, and so far 
as estimating age was concerned yield misleading results. 
As will be observed in the section dealing with physical condi- 
tions, not all bodies of water present the extreme conditions in relation 
to mussel life as those possessed by the rivers. Conditions are seen 
to be fairly uniform in Lake Erie, and such could be conceded to ~ 
favor uniform conditions of growth. This being the case, the num- 
ber of rings of growth on the shell could be reasonably conceived 
to represent the number of years the animal has lived. 
