The leathery black egg case of a skate is a common find along the beach.  Skates are non-bony bottom dwelling fishes related to rays and sharks.  They cruise along the ocean floor and eat mollusks and crustaceans.  The female lays the egg case partially in sand, but the case's long thin appendages can attach to seaweed or other flotsam. 

The egg case, also known as a sailor's purse or devil's purse, is originally waterproof, but after three weeks, the skate embryo develops gills.  Salt water eventually invades the case, and the small skate swishes his tail about inside to keep oxygenated water flowing in and out. 

After a few months, the baby skate finds a weak point in the lining and emerges into the open sea.