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BULLETIN OF THE BUREAU OF FISHERIES 
length than the shells from Arkansas and Iowa. Thickness studies emphasize this 
difference. 
Combining the data on length, weight, and thickness, it was found that the 
yellow sand shell requires about 5 years to reach a fair commercial size in the Missis- 
sippi at Fairport, Iowa; 4 years in the White River, in Arkansas; and but 3 or 4 years 
in the lower Rio Grande. 
Differences in the rates of growth of the two sexes of yellow sand shell, apparently 
correlated with the changes in the contour of the shell accompanying sexual maturity, 
were noted in each of the three localities. 
Considering the commercial aspect of length, weight, and thickness, together with 
the age of the year class, the yellow sand shells from the White River in Arkansas 
made the most satisfactory growth. 
Growth curves for the Pepin mucket, the buckhorn, and Pope’s purple were of the 
same general types as those described for the yellow sand shell. 
The Pepin mucket and the buckhorn grow more slowly and have longer life 
spans under the existing conditions in the upper Mississippi than the yellow sand shell 
in any part of its range as studied. 
Comparisons of valve areas — that is, the surface available for the cutting of 
blanks from the shells of the four species studied — -made on the basis of year classes, 
favored both' the rapidly growing species and the southern habitats. 
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