144 THE AMERICAN MIDLAND NATURALIST 
Howard and Clark, (2), it is evident that my data checks only with 
their observations for the Quadrulas, the average of growth for the 
remaining groups being considerably less. 
VI. CorreELATIONS. 
From the tables of percentages calculated for each species, the 
following correlations may be derived as to the growth of the 
different regions of the juvenile shell: 
1. Greater degree of inflation, DSD is associated with less dorsal 
ventral diameter, (DVD), (height), in all the species. 
2. In all but three species, (to which there are exceptions), there 
is a greater degree of inflation, (DSD), associated with greater pos- 
terior development of the shell, PD. The exceptions are Pleurobema, 
Ligumia, and Anodontoides. 
8. In-all but “three species, (to which there are ~ exceptions 
greater PD is associated with greater PHL. 
4. Greater DSD, PD, PHI and less D/VD are associated with 
less 7'H in all species excepting Pleurobema, Elliptio, Lasmigona, 
Anodontoides, Ligumia, and Lampsilis siliquoidea, ‘ 
5. These correlations of growth in juvenile shells are found to 
be very similar to those obtained from measurements of adult 
shells; (7, p. 173): 
VII.—Conc.usions. 
In addition to the data already presented as to the rate of growth 
in the different species, we may add the following conclusions to 
be derived, viz.: \ ; 
1. Fresh water mussels dwelling in lakes, like their river rela- 
tives, grow most rapidly in the earlier years of their lives, and the 
process of growth slows down considerably with advanced age. It 
must be remembered in this connection that none of the shells reported 
upon had reached an extreme old age. 
2. The rate of growth is highly variable, even in mussels of the 
same species. ‘ 
3. With the possible exception of the mussels of the Quadrula 
group, lake dwelling mussels as a rule grow more slowly than those 
from rivers, at least with regard to the dimension of length. 
4. The growth of juvenile shells of the species discussed exhibit 
mostly the same correlation in growth of parts as do the adult shells, 
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