736 
BULLETIN OF THE BUREAU OF FISHERIES 
portion. If the same type of variation of growth rate holds for the buckhorn in its 
northern and southern ranges, the buckhorn may be listed as having the slowest 
growth rate of any of the four species studied, as its actual growth rate at Fairport, 
Iowa, was about the same as that of the Lake Pepin mucket at Cross Lake, Minn. 
AREA OF SHELL 
Although length, weight, and thickness furnish indices of growth from which 
growth curves giving the physiological aspects of the problem may be constructed, 
from a practical commercial standpoint the growth of the shell is measured largely 
by the area of the shell from which button blanks may be cut. 
Since even the shells of the very flat species, as the pink heelsplitter, Propter a 
alata, are actually curved plates presenting both concave and convex surfaces, it is 
rather difficult to measure exactly the surface area of a mussel shell. For purposes 
of comparison, however, a projection was chosen, as in the cutting of blanks the 
shell is handled to a large extent as if it were a plane surface. Placing the valve on a 
piece of white paper, and tracing around its margin with a pencil, a projection of the 
shell was readily obtained. The area of this figure was then measured in square centi- 
meters by tracing the outline of the projection with a planimeter. 
The average values for the areas of the projections of shells from the various 
year classes are listed in Table 16. It is evident from this table that the surface 
of the yellow sand shell is greater for each year class in specimens from the southern 
portion of the range of this species than from the northern part. There is also a notice- 
able difference in area of the shell correlated with the sex of the individual. 
Considering shell area alone in terms of year classes, the comparisons of the 
four species examined favor the rapidly growing species and the southern habitats. 
Table 16. — Average areas (in square centimeters ) of valves by year classes 
Species 
Sex 
Year classes 
Locality 
II 
III 
IV 
V 
VI 
VIII 
XI 
Yellow sand shell, Lampsilis ano- 
dontoides. 
Do 
23.0 
28.8 
40.8 
Mississippi River, Fairport, Iowa. 
Do. 
41.3 
Do 
Male. 
20.7 
38.0 
39. 1 
White River, Newport, Ark. 
Dn ___ 
Female . 
40.8 
Do. 
Do 
Male 
23.9 
12.3 
44.8 
64.2 
Rio Grande, near Mercedes, Tex. 
Do 
35.9 
59.8 
Do. 
Lake Pepin mucket, Lampsilis 
siliquoidia pepinensis. 
19.9 
24.5 
38.3 
Cross Lake, Minn. 
26.5 
Mississippi River, Fairport, Iowa. 
Rio Grande, near Mercedes, Tex. 
37.4 
67.2 
DISCUSSION 
Considering all of the data obtained from over 1,100 specimens, the application 
of the annual-ring method to growth studies of fresh-water mussels seems both re- 
liable and practical, judging from the uniformity of the growth curves developed from 
these data and the ease with which these measurements may be made on the great 
majority of shells. 
That the shell rings are the result of an interruption of growth has been estab- 
lished experimentally (Lefevre and Curtis, 1912; Coker, Shira, Clark, and Howard, 
1921); and that the changes in environmental conditions during the winter months 
