ERESH WATER MUSSEL SHELLS 145 
VIII.—Svueaaerstions as tro Causes or Facts. 
Allusion has been continually made to the depauperate quality of 
many lake shells, so for the most part the results as obtained in the 
“paper” and Lampsiline groups of shells are those which would 
have been expected. In a previous paper, (5), I have been able to 
show that the thickness of shell of mussel species from L. Erie is 
negatively correlated with the percentage of calcium carbonate in 
the water, whereas in the Upper Ohio River Drainage, a positive 
correlation exists. L. Erie water, besides containing more calcium 
carbonate, contains greater quantities of other alkaline substances. 
The substances mentioned appear to act in such a way as to prevent 
less absorption of calcium carbonate by the L. Erie shells than by 
those of the Upper Ohio; as a consequence the L. Erie shells are 
thinner. 
It has also been previously stated that a convenient index to the 
rate of growth of a mussel lies in the rate of growth of its shell. 
Although length has been the common dimension by which the rate 
of growth has been estimated, and while thickness of the shell is 
the characteristic which may be associated with the amount of cal- 
cium carbonate in the water, it seems plausible to assume that the 
inorganic salts in L. Erie water may through favoring the develop- 
ment of a thinner sheil, cause a corresponding inhibition of the 
development of other parts of the shell architecture, the reaction of 
-the mussel being to maintain the “biological balance’’ of the different 
parts. This seems best illustrated with the results of the Quadrulas 
in L. Erie. They are always slow growers, and develop a thick 
shell. However, in L. Erie they have a thinner shells, so if growth in 
length is in pace with that of the shell, it seems to be at the expense 
of thickness. Under this combination of circumstances, the shell 
may take from the water, just what it needs in the way of lime—a 
point which is in accord with our knowledge of the absorption of 
inorganic salts by living shells. 
An interesting example in connection is that of Lampsilis, (luteola) 
siliquoidea which in Lake Erie develops into a thin and dwarfed 
‘shell. Yet in Lake Pepin, an expansion of the Mississippi river in 
southern Minnesota, which possibly presents all lake characteristics 
cited for L. Erie, except the high mineral content of the water, this 
species reaches its maximum size and development. 
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