1 6 HiCKLING, Variation of Planorbis multiformis, Bronn. 



termed " post-carinate," might theoretically occur, viz. : — 



(5) Post-carinate discoid. 



(6) Post-carinate turreted. 



Of these six types, four are actually realised among 

 the adult shells here described, types i and 5 being 

 absent. The first type occurs among the young shells, 

 but never retains its primitive form to maturity — in other 

 words, the unmodified ancestral form has ceased to exist. 

 The absence also of the fifth type seems to indicate an 

 antagonism between a discoid form and smoothness, 

 which may explain the non-survival of the primitive form. 

 Possibly the uncarinate discoid condition is mechanically 

 weak. 



In this connection, a peculiarity of the variation-curves 

 for height may be considered. It is clear that the curves 

 for the adult shells are asymmetrical. While the greatest 

 number of individuals have a moderately high spire, the 

 proportion of those with low spires, and especially of the 

 discoid types, is notably higher than it should be on 

 the hypothesis of pure simple variation about the mean. 

 In fact, the curve shows a tendency to rise again on 

 reaching the perfectly discoid forms, though unfortunately 

 the number of shells available is much too small to make 

 this peculiarity of the curve certain. The point is worthy 

 of further investigation with more material, since it would 

 indicate that, although the series of forms here considered 

 is undoubtedly continuous, there is a tendency for the 

 discoid type to separate off as a distinct form. However 

 that may be, there can be no doubt as to the general 

 asymmetry of the curves, which may best be translated 

 into the statement that there is a much greater variation 

 in height among shells with spires lower than those of the 

 commonest type than is found among shells of greater 



