Mermaids Purse
It would be nice to find these occasionally in fish traps and washed up on the beach (sea shore).
Like squirrel's stashes, Turkey and Argo gizzards and birds nests we could dissect them/open them to see what they contained, chances are you will find seaweed, rocks and sand but the possibility for rusty coins and silver, maybe even some gold flakes, those mermaid's do like shiny things.
IMO all of these things that can be rummaged/dissected, should be inspirationals, so you have to choose between the chance for something better or just studying them as they are.