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Vmversity of California Publications iii Zoology [Vol. 20 



While taking the measuremeuts of each individual, observations were 

 made regarding the position of the mouth, the type and size of macro- 

 nucleus, number and location of contractile vacuoles, and any other 

 characters which might aid in differentiation. 



In the handling of the data on dimensions I have followed in a 

 general way the method used by Jennings (1908) in differentiating 

 races of Paramiaecium. For the pui-pose of this work, however, the 

 results seemed more lucid if, instead of plotting length and breadth 

 along separate axes, the ratio of length to breadth was computed for 

 each individual and if these ratios were then plotted on the abscissa 

 wliile the numbers of individuals having each of these ratios were 

 plotted on the ordinate. In computing the ratios the quotient was 

 carried to the second decimal place. But in the construction of the 

 curves only intervals of tenths (or first decimal place) were used; 

 thus, for example, all ratios occurring between 1.25 and 1.34 inclusive 

 were grouped as if thej' were 1.3. This had two advantages : first, it 

 produced a smoother, steeper curve than would result if smaller inter- 

 vals were taken, using the same number of individuals measured, and 

 emphasized group rather than individual variations. Second, this 

 grouping reduced any error which might result from the observer 

 showing a preference for one graduation of the micrometer when 



