I found a bunch of these on the beach and saved them for my nature collection. I knew it was an egg case but not sure whose... Wikipedia describes it as 'a casing that surrounds the fertilized eggs of some sharks, skates and chimaeras'. Judging from photos in the article, this one is from a skate. Also known as a mermaid's purse or devil's purse, these egg cases usually wash up on shore empty, hollow, the young fish having already hatched out. Supposedly these egg cases are commonly seen washed up by the sea, but not on any of the beaches I've been on!