All fish are covered with overlapping scales, and each major group of fish has its own special type of scale. Teleosts ('modern' fish) have what are called "leptoid" scales. These grow in concentric circles and overlap in a head to tail direction like roof tiles. Sharks and other chondrichthyes have "placoid" scales made of denticles, like small versions of their teeth. These also overlap in a head to tail direction, producing a tough outer layer. Shark skin is available for purchase as shagreen, a leather which as original is smooth in one direction, and rough in the other direction. It may be polished for use, but is always rough in texture and resistant to slipping.