While Andrew is technically correct,  i think the original question
referred to shell-bearing molluscs in a more natural setting.....
anyway, as long as you aren't planning to nab large numbers of edible
shell-bearing members of the mollusc phylum, there are absolutely no
"regulations" that would prevent you from collecting any shell of any
mollusc at any time or any place except for national and provincial
parks.  There are no rare or endangered species of fresh water, land,
brackish or marine persuasion, and even if there were there would be no
need to protect them from the hordes of rabbid shell collectors that
avoid our shores and
woodlands/bogs/fens/marshes/streams/rivers/lakes/ponds/runs/salt
marshes/other wet places nearly every chance they get.
        At any rate, if anyone is up my way, my number is 876-2241, and i'd
love to show you around my corner of this little piece of heaven we like
to call New Scotland - except for the Latin-speaking people who
pronounce it Nova Scotia...

From somewhere near the edge of the planet,
Ross M.