FRESH WATER MUSSEL SHELLS 133 
Previously, this and other data had been gathered while making 
a study of the morphological characteristics of mussel shells inhabiting 
Lake Erie, (7), and there but remained the task for the present paper 
of regrouping the data chronologically, when percentages indicating 
the rate of growth based on the more commonly taken measure- 
ments of the shell 
length, height, and inflation, as well as others not 
commonly made, could be calculated. A detailed description of the 
method of making these measurements is given in the paper cited, but 
for the convenience of the reader they are stated to consist of the 
following: 
1. Length, £. 2. Dorso-ventral diameter, (hetght)) \ DY Dies 
3. Dextro-sinistral diameter, DSD, (breadth). 4.5. Distance an- 
terior, (AD), and posterior, (PD), from DVD, to extremities of 
valve, the sum giving the total antero-posterior dimension. 
6.7. Length of anterior, (AHL), and posterior, (PHL), part of 
hinge measured from the beak, giving total hinge-length. 8. > Thick- 
ness of valve just superior to the pallial line in the region beneath the 
umbo. The measurements and calculations made from them con- 
stitute the results. 
. V.—REsucts. 
In the following tables, each species is first dealt with separately. 
The ages indicated were obtained by counting the lines of growth 
on each specimen. In the two columns to the left are indicated the 
number of specimens used for an estimated age. The remaining 
columns contain first, the mean of the measurements described. above 
and expressed in centimeters for the youngest juvenile of the species 
available. The rest of the columns consists of the mean of the dimen- 
sions of juveniles and mature shells of succeeding ages expressed in 
percentages of the corresponding dimensions of the youngest juvenile. 
The comparisons, etc., following the tables of percentages for each 
species are based most largely on the average of these percentages 
for the probable number of years represented by the series of material 
of that species. At other times, however, conclusions taken are based 5 
upon direct inspection. All percentages expressed are an index to 
the amount of growth in a particular part of the shell for the time 
stated. Finally, for the convenience of both reader and writer in 
making a comparative study of the rate of growth in different species, 
we 
