I have been using egg shells (chicken or goose) for Anguispira alternata &
some unidentified polygyrid snails. I either crush them or place one or two
large pieces in the snails' container. I have had the chance to observe an
Anguispira while it was consuming pieces of egg shell. The snail was upside
down on the transparent lid of a plastic container. I sprayed the lid with
water, then sprinkled crushed egg shell on it. While I was watching the snail
under my stereo microscope, I saw it manipulate in its mouth & swallow
several tiny pieces of egg shell. Lately, I place large pieces of egg shell
in the containers. On several occasions I have seen snails sitting on top of
the shells.
 
Here is a recipe from an old paper about British snails (Boycott, J. Ecology,
22:1, 1934): "Mollusca of very diverse natural habitats (e.g. Helix.......
and Geomalacus) will all flourish and breed well through successive
generations on a mixture of fine oat meal and powdered chalk spread on
cabbage leaves, and no snail or slug to which I have offered the mixture has
had any hesitation in eating it."
 
A.