In Cassis cornuta, the horned helmet, one of the largest gastropods,
the male shell has a row of several large blunt spines around the
shoulder of the shell.  The female has a larger number of much
smaller, more pointed tubercles.
In Lambis lambis, the common spider conch, the lip spines of the
female are markedly upturned, while those of the male protrude almost
straight out from the lip.  This (I have read) allows the shells to
fit together better during mating.
In Viana regina, a Cuban land snail, the male has a deep indentation
or notch in the lip, while the lip of the female is straight, with no
sign of such an indentation.
Question:  How many cases of dimorphism exist where no-one has yet
checked to see if it might be gender-related?
Paul M.
Rhode Island