Aydin,
 
Thanks for the reference.  Other references to related literature on
molluscan variation is welcomed.
 
Rich
 
At 09:50 AM 2/3/98 EST, you wrote:
>While waiting for Rich's answer, I may recommend the following: C. Little &
>J.A. Kitching, 1996 "The Biology of Rocky Shores" Oxford U. Press. This a
>neat little introductory book written for college students and independent
>investigators with plenty of references to the original sources. Although it
>is not about molluscs per se, most of the discussions are centered on them.
>One section pertinent to the ongoing discussion is titled "Variation in shell
>shape". Here the authors conclude: "Conditions for genetic differentiation
>are probably most propitious for intertidal animals that lack any pelagic
>stage, such as Nucella. They tend to stay isolated in localised populations,
>and their genetic condition is preserved."
>
>A.
>
>