136 THE AMERICAN MIDLAND NATURALIST 
in life. Isley also records, (p. 11), the growth of other specimens of 
this shell, the smallest of which most closely corresponds to the oldest 
of my material. Here the rate of growth obtained in the 3 dimen- 
sions in order is 15%, 10%, 11%. Using the shell dimensions 
composing the means for 15 and 16 years, I obtain 11%, 12% and 
14% which allowing for environment seems fairly close. Coker, 
Shira, Howard and Clark, (2), studied Quadrula plicata, a closely 
related species. They report it 13.5 mm. in length at the close of 
the second year as reared in a pond. This is nearly a mm. smaller 
than my youngest specimen of plicata. They, however, recognize 
the fact that conditions of pond life are apt to produce stunting 
effects. They record another specimen which increase 5.17 in. in 
2 yrs., 3 mos., 2 weeks. I find a similar increase in my specimens 
of costata between the ages of 2 and 4. 
I find I can select a series of variants which will exhibit very 
nearly the same percentage of yearly increase as Iseley’s remaining 
data on Q. undulata, (p. 13). Isely found a specimen 48 mm. in 
length increased 13% in that dimension in one year; a 64 mm. 
specimen, 12% ; a 70 mm. specimen, 107% ; 72 mm., 108%; 82 mm., 
103%. The ages of my material to which these correspond are 
5, 9, 10, 10, and 14 years. 
My results, as obtained by the method described, indicate that 
Isely’s estimate of a yearly growth in length of from 5-25 mm. is 
somewhat conservative for one based on the smallest and youngest 
specimens. I find that by applying the average growth in length, it 
is at least 31 mm. in lake shells. Within the limits of my material, 
it seems probable that a length of 110-120 mm. could be reached at 
15 years, as Isely found in the case of mussels from Shoofly Creek. 
Undoubtedly the environment is a very large factor in determining 
the rate of growth. 
3. PLEUROBEMA COCCINEUM PAUPERCULUM 
No. 
Spec. Age L DVD DSD PD AD PEE ARs sins 
2 8 5.84cm. 4.47cm. 2.85cm. 4.8 cm. .97cm. 3.12cem. 1.28cm. .27cm. 
2 9 105% 106% 103% 114% 140% 112% 103% 125% 
2 10 114% 110% 120% 123% 114% 121% 146% 135% 
2 Ik 195%. 115% 109%. 125% 174% . 131% WlGort74gy 
2 12 119% 97% 111% 114% 160% 111% 104% 77% 
Average growth per year in each dimension for 12 years, (oldest shell), 
6% 2.5% 4% 64% 1%% 64% 964% 
Average growth per year in each dimension for 5 years, (8-12 yrs. inclus.) 
1% 6% 9% 15% 18% 15% ISG a 22 oe 
e 
