BOYCOTT : THE RADULA OF MYALtNlA. 23I 



Number of Rows ... 

 Number of Marginals 

 Teeth in one row 

 Teeth in Radula 

 Length of Radula — mm. 

 Width of Radula — mm. 

 Area of Radula — mm. - 

 [Major diameter of shells 



The significance of these figures is this. The range of normal^ 

 variation in biological sizes is such that practically all individuals are 

 smaller than the mean plus three times the standard deviation and 

 larger than the mean minus three times the standard deviation. Nine- 

 teen out of twenty normal individuals differ from the mean by less 

 than twice the standard deviation, and about two-thirds by less than 

 once the same figure. If then the standard deviation were 5% of the 

 mean — if, in other words, the coefficient of variation were 5 — we 

 should expect to find specimens pretty quickly which varied from the 

 mean by 10% and we should regard as probably abnormal or belong- 

 ing to another category of classification any individuals which differed 

 from the mean by more than 15% or thereabouts. 



The following will serve as examples of the distribution of the values 

 under consideration : any of the others may be readily extracted from 

 tlie tables. 



In each instance it will be seen that all of the 66 individuals come 

 within the range of the mean 1 3 standard deviations. In the 

 case of the number of rows one, in the number of teeth three and 



I i.e., "Spontaneous" variation not due to any ascertainable factor such as locality, etc. 



