No. 520] VARIATIONS IN UROSALPINX 581 



row, although varying in size, fall into a single form- 

 group as determined by the ratio between total height 

 and the greatest shell-aperture. A measuring machine 

 such as that used by Bumpus for his work on Littorina 

 made it possible to read the ratio of the two dimensions 

 directly from a graduated arm without trouble of com- 

 putation, thus greatly lessening the tediousness in ob- 

 taining the data. 



3. Are Variation Curves of any Locality Distinctive 

 for that Locality?— Tests were first made to ascertain 

 how far the personal clement in making the measure- 

 ments could affect the results, since judgment in the use 

 of calipers and in the manipulation of the measuring ma- 

 chine are by no means invariable factors. One such 

 test, which is typical of several which were made, is 

 shown in Table 1. where the same lot of shells was twice 

 measured. 



from exact similarity represents the imperfections of 

 manipulation and it will be seen that according to this 

 test a difference of .096 in the standard deviation with 

 a probable error of about ± .03 may be regarded due to 

 imperfect technique. 



Now in order to test whether the variation is charac- 

 teristic and constant for any locality whence the shells 

 came, two 1,000-lots were gathered on the same day in 

 1898 from the same restricted group of rocks on Nobska 

 Point, Woods Hole, by no means thereby exhausting the 



