8 HiCKLING, Variation of Planorbis multiformis, Broftn. 



T he variation in the rate of assumption of the turreted 

 condition may be seen best of all by cutting the shells in 

 median section. The adjoining Figs. ^ and 5 are camera- 

 lucida drawings of the apical portions of two such sections 



Fig. 4. Section of apex of shell of mean height, 

 showing smooth discoid young stage 

 with wide umbilicus. 



Fis;. 5. Section of aj^ex of shell of mean height, 

 showing rapid assumption of turreted 

 character and early narrowing of um- 

 bilicus. Compare with Fig. 4. 



of shells which were of exactly the same height.* The 

 contrast between the form of the young stages, at the 

 completion of the third whorl, is extremely striking, 



*The shells were exactly similar to those of Pi. //., Fig. 5 a, b. Fig. 6 

 is a section corresponding to PI. II., Fig. 5 c. 



