ANNUAL GROWTH OF FRESH-WATER MUSSELS 729 
Table 9. — Increase in length, weight, and thickness of yellow sand shell, Lampsilis anodontoides {Lea) 
MISSISSIPPI RIVER, IOWA 
Annual ring 
Male 
Female 
Length 
Weight 
Thickness 
Length 
Weight 
Thickness 
Centi- 
Per 
Per 
Centi- 
Per 
Centi- 
Per 
Per 
Centi- 
Per 
meters 
cent 
Grams 
cent 
meters 
cent 
meters 
cent 
Grams 
cent 
meters 
cent 
I 
1.61 
15 
1. 58 
15 
II 
3.93 
53 
3.59 
49 
III 
2. 68 
78 
15.3 
3G 
0.25 
83 
3.30 
81 
IV 
1.35 
91 
29.7 
71 
.35 
117 
1.49 
95 
V 
.81 
99 
42.5 
101 
.40 
133 
.87 
103 
46.9 
112 
0.45 
150 
Total 
10. 38 
| 
10.83 
1 
1 
WHITE RIVER, ARK. 
I 
2.80 
27 
3.35 
32 
II 
3. 89 
64 
3.58 
66 
III 
2.24 
85 
20.7 
49 
0.30 
100 
2. 20 
87 
33.9 
81 
0.35 
117 
IV 
1.33 
98 
40.5 
96 
.40 
133 
1.30 
99 
48.0 
114 
.40 
133 
V.. 
1.24 
no 
45. 1 
107 
.45 
150 
.54 
104 
49.0 
117 
.45 
150 
Total . 
11.50 
10. 97 
RIO GRANDE VALLEY, TEX. 
I 
4.61 
44 
4.80 
46 
II 
3.41 
76 
11.04 
26 
3. 53 
79 
18. 08 
43 
III 
2.09 
96 
38. 64 
92 
0.30 
100 
1.87 
97 
26.19 
62 
0.25 
83 
IV 
1. 10 
107 
43.62 
102 
.35 
117 
1.90 
115 
38.00 
90 
.30 
100 
V 
.79 
114 
53. 55 
127 
.40 
.90 
124 
Total.. 
12.00 
13.00 
As might be expected, after the consideration of the relative weights of the shells 
from the three localities since thickness is in a way a function of weight, the meas- 
urements of shell thickness obtained (see Table 9) show the shells from the Mississippi 
River in Iowa and the White River in Arkansas to be definitely thicker than those 
from the lower Rio Grande Valley in Texas. The shells of the females from the 
Rio Grande were found to be thinner than the males of the same year class, again 
paralleling the weight-age data from this locality, although the females equalled or 
exceeded in thickness the males of the same year classes in both the White River 
and Mississippi River collections. It is obvious that from a commercial standpoint, 
the shells must not only be of a good texture and size, but must be of suitable thick- 
ness; that is, neither too thin nor excessively thick. As has been pointed out in a 
previous section, shells having a thickness of 0.30 centimeters or a little more, are 
of a desirable thickness. This thickness is attained by the females in the Rio Grande 
Valley and by the males and females in the White River in northern Arkansas by the 
end of the third year, but not until the end of the fourth year in the Mississippi at 
Fairport, Iowa. The annual increase in thickness in all year classes averaged about 
0.05 centimeter suggesting that the rate of increase in thickness is a relatively con- 
stant factor. As, however, these measurements were taken in the region of the 
pallial line, these observations concerning the thickness of the shell and the annual 
increment of thickness can not be extended to other parts of the shell until additional 
data have been collected. 
