A sea cookie is a type of sand dollar — specifically, it’s the common name often used for certain species in the genus Encope (and a few related ones). Like other sand dollars, sea cookies are flat, disk-shaped echinoderms related to sea urchins and starfish, but they’re typically a bit thicker and more rounded than the classic sand dollar you might find in a shell shop. When alive, they’re covered in short moving spines and are surprisingly quick burrowers — but once dead and cleaned, they look like a delicate, patterned coin from the sea.