i think most clams you'll find for aquariums will either be cold-water or saltwater species, so neither will work in a tank with a betta. snails, however, there are a lot of very nice snails that would be wonderful. apple/mystery snails, assassin snails, MTS and ramshorns are all nice options.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)