Perhaps the similarity of the shells in these two groups is not so much
mimicry of one group by the other, as similar adaptations related to a
similar life style (burrowing under the sand).  One point that has not been
mentioned - are these two groups believed to have common ancestry?  Or are
they believed to have resulted from relatively unrelated lines which
gradually came to resemble each other in shell structure, like Patellidae and
Siphonariidae (what would be called "convergent evolution")?
On the question of how dogs which differ greatly in appearance can all be the
same species - let's remember that these breeds are not products of nature.
They are the products of genetic engineering by humans.  By selective
breeding for certain characteristics, forms can be produced which would never
have occurred in nature, and changes which could take millions of years by
natural means can be brought about in a few hundred years or less by
controlled breeding.  If we could selectively breed olive shells the way we
breed dogs, we could no doubt produce forms of a species that are as
different from one another as a mastiff and a dachshund.  But they would not
necessarily be very different genetically, since the genes that were selected
by such breeding are probably relatively few.  And, we would probably find,
as with dogs, that all the forms are fertile with one another, and can
produce offspring with intermediate characteristics.
Paul M.