I finally got the shell collection complete. I still need one butterfly (uncommon), one beetle (rare), and three pebbles (two rare and one uncommon).

I don't know if it was a coincidence or what, but I got the last seashell - actually the red starfish - after all my villagers got a plague caused by a dead parrot - and made sure every last one of them got healed. It took awhile as about 80 out of 90 people were sick, but I managed to make three adults into trainee doctors and used two kids with healing skills as well.