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THE AMERICAN NATURALIST [Vol. XLIV 



7. Variations due to Age. — It is indeed true that as 

 the snail grows older not only is there a change in the 

 total height of the shell, as would be expected, but also 

 the ratio of the largest shell-aperture to the height 

 diminishes in a definite way and the standard deviation 

 becomes generally less. In other words, the older the 

 shell becomes the less is the relative size of the largest 

 shell-aperture to the total height and the less does it 

 tend to deviate from the arithmetical mean. In Table 

 VII the total number of shells measured in 1898, 1899, 

 1900 are arranged according to their height to illustrate 

 this fact. 



8. Staten Island and California. Shells Compared.— 

 In 1900 shells were obtained from several additional lo- 

 calities, among which were 1,6(55 from oyster beds on 

 Prince's Bay, Staten Island. This lot of shells has a 

 special interest because it was from this particular lo- 

 cality, according to Dr. H. M. Smith, of the U. S. Fish 

 Commission, that the oysters, and accidentally with 

 them the Urosalpinx, were obtained for transplanting 

 to San Francisco in 1871. A comparison of the Staten 

 Island shells with the California shells appears in Table 

 YITT. 



TABLE VIII 



From this table one of three conclusions must be 

 drawn: (1) That the introduced California shells vary 

 less in their new environment than they did in the place 

 they came from or (2) that the Staten Island shells have 

 increased remarkably in their variability since 1871, or 

 (3) that place-modes in which time element is not known 

 are of little value in working with organisms of this 

 kind. 



9. Shells of Successive Years Compared.— Further- 



