Manchester Mciitoiis, Vol. hii. (191 3), No. 10. 



was made for each of the three grades of size. Since it 

 is almost impracticable, as already indicated, to obtain 

 numerical values for the character here considered, the 

 successive grades have merely been numbered A to J in 

 order of increasing height-ratio. In the subjoined curves 

 {Fig. i) the results of this enquiry are plotted, height- 

 ratios being taken as abscissae, numbers of individuals as 

 ordinates. 



C D C F G " I 



I. Curve showing distribution of "height" 

 among 533 individuals 



Some of the indi- 

 heights 



height 



V diameier of base, 



of Planoi-bis inultiforniis. 



viduals were not full-grown. Th 



represented by the letters A-J may be seen by 



reference to PI. I., Fig. i. 



The Variation Curves : Biological unity. In Fig. i is 

 shown the curve representing the frequency of the "height" 

 character in all the shells (533) examined. The group A 

 includes the " lowest " shells (those actually discoidal), 

 while group J represents the " highest " forms. The type- 

 set of shells is shown in PI. /., Fig. i. Inspection of the 

 curve at once reveals the fact that the numbers of indi- 

 viduals falls off very regularly as the height of the shell 



