Corystes cassivelaunus, the masked crab, helmet crab or sand crab, is a burrowing crab of the North Atlantic and North Sea from Portugal to Norway, which also occurs in the Mediterranean Sea. It may grow up to 4 centimetres or 1.6 inches long (carapace length). The name "masked crab" derives from the patterns on the carapace which resemble a human face, in a similar manner to heikegani.