When your walking along the beach you never know what you will find. Hidden treasures are lurking in the sand. You might see a sand dollar, sharks tooth, a starfish, but if you are lucky you will find a Mermaid Purse.
A mermaid purse is an egg case. Inside you could find the egg of a Shark (Bullhead Shark, Carpet Sharks, or Ground Sharks), or Skates (Big and Longnose). https://en.wikipedia.org/wiki/Egg_case_(Chondrichthyes)