ANNUAL GROWTH OF FRESH-WATER MUSSELS 
727 
Before comparing the weights of the shells for the three localities, attention is 
called to Figures 11 and 12, which give the mean weights for males and females, 
respectively. The weights of individual valves are shown by the black squares on 
the right hand side of their respective year classes, while the mean weight for each 
class is shown by a square with diagonal lines to the left of the year class. 
Table 7. — Mean weight of left valves of yellow sand shell, Lampsilis anodontoides (Lea) 
MALE 
Mississippi River, Iowa 
White River, Ark. 
Rio Grande Valley, Tex. 
Ring No. 
Number of 
specimens 
Weight 
Number of 
specimens 
Weight 
Number of 
specimens 
Weight 
II 
Grams 
Grams 
6 
Grams 
11.04 
III 
4 
15.3 
3 
20. 7 
14 
38. 64 
IV 
2 
29. 7 
8 
40. 5 
28 
43. 62 
v__ 
23 
42. 5 
5 
45. 1 
2 
53. 35 
VI 
19 
54.0 
2 
73.0 
1 
92.20 
VII 
2 
71.5 
2 
96.7 
VIII 
1 
100.1 
FEMALE 
II 
8 
18. 08 
III 
1 
33.9 
9 
26. 19 
IV 
6 
48.0 
4 
38. 00 
V 
20 
46.9 
15 
49.0 
VI 
VII 
22 
6 
56.0 
64. 6 
2 
58.2 
i 
57.10 
VIII 
2 
77.9 
Returning to the consideration of the weight values (as shown in fig. 10 for all 
three localities), there seems to be a tendency toward greater weight for the males 
than for the females of the same age in spite of the fact that the graphs of increase 
in weight are much more irregular than those of increase in length, presumably as 
the result of the smaller number of observations made on weight. This trend toward 
greater weight for the males is more evident in the older shells, becoming apparent 
after the fifth year for the shells from the Rio Grande Valley, and after the sixth 
year for the shells from the Mississippi River, with the shells from the White River 
as intermediate between the two. It is probable that the lighter weight of the female 
shells may be corolla ted with the reproductive activities of the female; that is, the 
calcium and other metabolic demands of the glochidia while developing from the eggs. 
If the weight and length of the shells be considered together, the shells from the 
Rio Grande Valley are found to be definitely lighter for any given length than the 
shells from either the Mississippi in Iowa, or the White River in Arkansas. This is 
particularly true of the females from the Rio Grande which have weight values in 
the different year classes markedly lower than those of either sex from either of the 
other two localities. Commercially, the button manufacturers reject the smaller 
yellow sand shells from the Rio Grande Valley as being too thin for use, but accept 
those of the same linear dimensions from the White and Mississippi Rivers. This 
fact is readily understood when it is remembered that the 2, 3, and 4 year old shells 
from the Rio Grande district are definitely'longer than shells of the same ages from 
either the White or Mississippi Rivers. The rapid growth in length made by the 
Rio Grande shells seems to be made in part, at least, at the expense of gains in shell 
weight. 
