Land snails can easily and successfully be kept in a terrarium - but not
with a box turtle!  Box turtles LOVE snails, earthworms, etc., as well as
berries, greens, and the like.  Their jaws are powerful enough to crunch
through the shell of a Cepaea nemoralis, which is one of the things I used
to feed box turtles back in my herpetology days.
Paul M.