Mermaid's purses (also known as Devil's Purses) are the normally empty egg cases of skates, sharks and rays. They are among the objects which are washed in by the sea, and can be found at the shore, at the furthest point of the high tide.
The size of mermaid's purses vary; those of the small-spotted catshark or lesser spotted dogfish, Scyliorhinus canicula, are around long, while those of the greater spotted dogfish, S. stellaris, are around . That excludes the four long tendrils found in each corner, which assist in anchorage. Mermaid's purses from rays vary in that they have points rather than tendrils. The colour and shape of mermaid's purses also varies greatly from species to species.
If a mermaid's purse is still moist (not dried out) and has no visible hole, it is probably still occupied.