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THE AMERICAN NATURALIST [Vol. XLIV 



supply. The figures for these two lots are shown to- 

 gether in Table II. 



TABLE II 

 Place- Modes. 



Difference | .0431 ,082j 



The close resemblance of these two lots, which show a 

 difference in standard deviation (.082) less than that 

 shown when the same lot is measured twice (.096) as just 

 indicated in Table I, warranted confidence in the proba- 

 bility that all the shells from a given place, when col- 

 lected at the same time, exhibit the same characteristic 

 sort of variation which may therefore be regarded as 

 distinctive for that particular locality. Furthermore, a 

 glance at Table III will show how widely the shells of 

 various localities may differ with regard to the character 

 and degree of their variability, a fact that assures us that 

 in Urosalpinx we are dealing with a form whose varia- 

 tion is considerable enough to furnish favorable mate- 

 rial for quantitative treatment. 



3 Formulas for standard deviation and probable error of standard devia- 

 tion are found in Davenport's Statistical Methods (Davenport, C. B., 1899), 



