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W. J. CROZIER 



than that measured for the total length (148). For the same 

 group the correlation based upon total length estimations is 

 r = 0.492 (table 3). This agreement (fig. 10) is sufficient to 

 show that the method of measuring 'total length/ as previously 

 described, is free from serious objection so far as the variable 

 length of the 'tail' of the foot is concerned. Even with small 



Fig. 9 Showing the relation between the "mantle-length" of specimens of 

 C. zebra (abscissas) and the average length of their mates (ordinates), for 89 

 pairs; (1) is the line upon which the observed points, (2), would fall if no cor- 

 relation were involved; m-w' is the mean for all. The smoothed regression line 

 is shown thus • — ■^- — . The class unit is 1 cm.; r = 0.518. 



numbers of this sort the correlation is entirely too high to be 

 explained as the result of assortive mating. 



B. In one small series of 66 pairs the volume of the animals 

 was measured. The frequency distribution of volumes is shown 

 in figure 12. The correlation index for these pairs, on a volume 

 basis, is low (r = 0.1353), as shown in table 4, and the distri- 

 bution of the measurements is very irregular (fig. 13). This 

 series is not extensive, yet the result of its examination has 

 some value. The volume of a Chromodoris may in part de- 



