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JOURNAL OF CONCMOLOGY, VOL. I4, NO. 8, OCTOBER, I9I4. 



in the length of radula three, individuals exceed the limits of twice the 

 standard deviation. For the number of rows and the number of teeth 

 in a row one would therefore expect to find commonly a variation of 

 about 10% on either side of the mean, and any single radula with 37 

 to 45 rows, or with 26 or 33 teeth in a row, could not be considered 

 as necessarily different in that respect from that of a Banstead helvetica 

 of about 7 '5 mm. diameter. The case would, of course, be quite 

 different if a series of radulae gave such a result as an average. The 

 necessity of examining a number of specimens in any case is apparent. 



The coefBcient of variation of a linear dimension — diameter or 

 altitude — of most snail shells is about 5, of their volume 12 or 15. It 

 appears, therefore, that the number of rows and number of teeth in a 

 row have about the same variability as the diameter of a shell. The 

 length and area of the radulse are apparently more variable. The 



