Well, I've boiled the shells (which turned them green!), then dried them.
Then bleached them (1/8th strength using "Thin Bleach" which made them 90% white again), then rinsed them in tap water, then dried them. Then rinsed them again, then soaked them in tapwater for a day.
They look ok, but they still smell a bit - I plan to rinse them thoroughly again and let them dry naturally.
If they still smell after this (they don't smell of bleach, just a "strange" baddish smell) what should I do??