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.