No. 526] 



VARIATIONS IN UROSALPINX 



579 



port) so that any lot would, as far as possible, be typi- 

 cally representative of its locality. Lots of 1,000 were 

 taken and shells not immediately measured were simply 

 preserved in formalin until opportunity for making use 

 of them arose. 



In ascertaining statistically the variability of any lot 

 of shells it was necessary to select for measurement two 

 easily definable dimensions common to every shell and 

 take the ratio of these two dimensions for reasons which 

 will directly appear. The dimensions selected were the 

 total height of the shell (a to b, Fig. 1) and the greatest 

 dimension of the shell-aperture (a 

 to c, Fig. 1). It was possible to de- rs— b 

 termine these standards on Urosal- 

 pinx by the use of calipers with a / 

 considerable degree of accuracy. [/ \ 

 Any other dimensions which would Y /\ 

 lend themselves equally well to ac- / / ] 



curate measurement would have V I j 



served quite as well to establish a \ I J 

 criterion from which a comparison N. 1 / 



of variability in different lots of \ 1/ 



shells could be computed, since it \J/._ a 



was the fact of variation, and not fig. i. fl -& = height of 

 the direction or character of it that JJJJJj u "~' = greatest mouth 

 was the object of the inquiry. 



The ratio of the two dimensions was used instead 

 of a single dimension in order to eliminate as far 

 as possible heterogeneity referable directly to growth. 

 ITad height alone, for instance, been used then groups 

 of shells would be related to each other with reference to 

 their variations in size or age only, and all that could be 

 said in comparing lots from two localities would be that 

 those in one locality averaged taller or shorter, and pre- 

 sumably, therefore, were older or younger than those 

 from another locality. This would not be a suitable index 

 for variation in form. On the other hand, when the ratio 

 of two dimension^ is taken, then the factor of absolute 



