While beachcombing it's not uncommon to come across the strange thing pictured above mixed in among the various beach debris. This is the empty egg case, or "Mermaid's Purse", of a Skate.
Fish are divided into two main groups: the cartilaginous fish, which include sharks, skates and rays, and the bony fish, which include nearly all of the rest of the fishes found in both fresh and salt water. The bony fish have a skeleton made of calcium, like those of birds and mammals. The cartilaginous fish have a skeleton made of cartilage.
Unlike most bony fish, which shed hundreds or even thousands of unprotected eggs into the water, cartilaginous fish produce only a few young and protect them inside a leathery egg case. Most female sharks keep the egg case inside their bodies and give birth to live young. Skates, rays and some types of sharks, however, lay a couple of egg cases on the ocean floor and leave them to develop alone.