134 THE AMERICAN MIDLAND NATURALIST 
the important data for each species is summarized in the table on 
“Comparative Growth of Species Studied,’ following the individual 
discussion of species. 
c 1. FUSCONAJA FLAVA PARUOLA 
oO. 
Spec. Age Ee. DVD “DSH PD AD "PHL AHE. > fH 
1 2 77cm. 1.3cm. .7cem. 1.3cm. .4cm. 75cm. .4cm. .13cm 
162997 330%'~ 410%. 317%. 250%, 333% 25097, 23004 
12 355% 347% 464% 400% 212% 500% 300% 292% 
verage growth per year in each dimension for 12 years, 
439% 150% 209% 161% 110% 192% 116% 87% 
3 
4 
4 
4 
4 
4 
4 
Z 
2 
2 
A 
DepucTIONS FROM TABLE OF PERCENTAGES AND REMARKS. 
In this species, increase in the various dimensions of the shell is 
greatest in later years. The increase in DSD, PHL and L is the 
greatest, 7'H the least. dD and AHL increase uniformly. From 
years 8-11 inclusive, DVD, DSD, PD, PHL, and AHL increase more 
rapidly than from 4-7 inclusive. The increase in thickness is less 
marked in later years, while that of AD is also low. 
The conditions under which Iseley, (10), did his work, probably 
prevented any attempt upon his part to estimate the age of the mus- 
sels used in his study. However, in the river form of this species, 
(Quadrula rubiginosa), he gives, (p. 10), the average yearly growth 
of 3 young specimens as an increase of 6.1% in length, 5.6% in height, 
5.39% in breadth. From p. 11, an increase of 3% in these dimensions 
in two other specimens can be inferred, while 9.2% gain in length is 
deduced from the records of two additional specimens given on p. 12. 
It is unfortunate that none of the juveniles I had access to had quite 
reached the dimensions of the shells he used, but my results check 
with other investigators in that the rate of growth diminishes with 
age. Had it been possible to make a comparison on size alone, it 
would have been seriously discounted by the fact that lake dwelling ~ 
shells are often depauperate. On the basis of the statement of 
authors cited, (2), that shells of the niggerhead and related species 
increase 14 in. more or less in a year, my averages indicate in con- 
ed 
