Thanks Andrew and Paul for the notes on sexual dimorphism in shells. I keep
notes about all the new stuff I learn. It doesn't seem like a big topic of
study in molluscs.  I also read that in the Unio that lives in the rivers near
where I live, the female is quite inflated and the male is flatter.  I have
some of these and the difference is very noticeable.
 
Andrew K. Rindsberg wrote:
 
> Among ostreid oysters, it is common for individuals to change sex several
> times during a lifetime. Young oysters are all male, but mature oysters may
> be male or female.
>
> The situation in the slipper shell Crepidula is somewhat similar, where the
> young are male and mature individuals tend to be female.
>
> Some ostreine genera (subfamily Ostreinae) are "incubatory", meaning that
> they incubate the larvae before releasing them (Stenzel, 1971). The shells
> commonly have no umbonal cavity, or only a very shallow one, and the left
> valve is less capacious. Even extinct species can be recognized as
> incubatory or nonincubatory.
>
> Incubatory genera include Ostrea Linnaeus, 1758; Cubitostrea Sacco, 1897;
> Ferganea Vialov, 1936; and others.
>
> Nonincubatory genera include Crassostrea Sacco, 1897; Saccostrea Dollfus &
> Dautzenberg, 1920; Striostrea Vialov, 1936; and others.
>
> I realize that this doesn't exactly answer the question, but it's as close
> as oysters get to sexual dimorphism.
>
> Reference:
>
> Stenzel, H. B., 1971, Oysters. In Teichert, Curt (ed.), Treatise on
> invertebrate paleontology, part N, v. 3, p. N953-N1224. Geological Society
> of America and University of Kansas, Boulder, Colorado. In print; available
> from Geological Society of America (see their Website for details).
>
> Andrew K. Rindsberg
> Geological Survey of Alabama (the "other" GSA)