K. Pearl and F. J. Diinbar 



323 



approximatcly plane ventral surface, containing in its centre the ciicular "inoiith' 

 opening. The form df iJic shell is shown in sidc vii'w in Fig. 1. 



FlQ. 1. 



Fio. 2. 



The points studied in the work vvcre : (i) the Variation of the diameter of the 

 shell (Fig. 2, ab), (ii) the Variation in the diameter of the " mouth " opening 

 (Fig. 2, xij), (iii) the Variation in colour, and (iv) the correlation between (i) and 

 (ii), and (i) and (iii). For verbal convenience vve shall refer throughout the paper 

 to character (i) (the total diameter of the shell) as the outer diameter, and to 

 character (ii) (the diameter of the " mouth'' opening) as the inner diameter. 



The measurements of the outer and inner diameters were made in the following 

 way. At the beginning of the taking of the series of measurements a Compound 

 microscope with a camera lucida attached was set up in a certaiu place in the 

 laboratory and adjusted so that both microscope and camera gave optically their 

 best results. The arrangement was then left undisturbed*. The amount of 

 magnification obtained by the use of microscope and camera with the paper at the 

 level of the table was then very carefully measured several times and the mean of 

 all these observations used as the true value of the magnification. 



* Except ouce tluring the eouise of the measuring, when it became necessary to use the microscope 

 elsewhere. Wlieii replaced it was adjusted so aa to be in the same condition as beforo the reraoval. 



41—2 



