Manchester Memoirs, Vol. lix. (191 5), No. 4. 15 



to those at the other extreme, in which the ventral beak 

 is remarkably large and prominent as compared with 

 that of the dorsal valve. This feature of variation is 

 illustrated by the pair of extremes/ and K, PI. I. 



Upon sorting the shells into groups representing those 

 with equal beaks, those with very unequal beaks, and 

 those with beaks representing the mean between the 

 extremes, no definite correlation was to be observed 

 between this feature of variation and any other shell 

 character. 



2. Lateral Ttuis ting of the Ventral Beak. 



This remarkable feature of twisting of the ventral beak 

 might readily have been overlooked in the handling of a 

 few individuals, as merely a chance variation, and only 

 became evident as a regular feature of variation upon an 

 examination of the shells as a body, when the large num- 

 ber of individuals possessing this feature became very 

 evident, the asymmetry produced in some cases being 

 very appreciable. 



Still more remarkable is the fact that twisting takes 

 place, in some individuals to the right, in others to the 

 left, with a greater proportion of left-twisted forms. The 

 asymmetry produced, affects not only the umbonal region 

 but also the form of the shell as a whole. 



On PL I., H represents the symmetrical type, whilst 

 G and / represent extremes of twisting to left and right 

 respectively, and between the mean and the extreme on 

 either side there are shells showing almost imperceptible 

 gradations in the amount of twisting to right or left. 



3. Ornament of Shell. 



The ornament of this species of Reticularis following 

 the characters of the genotype is reticulate, but there is 



