Dentin is a type of bone tissue that is part of the placoid scales of shark fish.
The flat, rectangular plate at the base of the placoid scale is embedded in the fish's skin. Like teeth, placoid scales have an inner pulp core consisting of connective tissue, blood vessels and nerves. The pulp cavity is nourished by a layer of odontoblast cells that secrete dentin. This hard, calcified material forms the next layer of scales, which fit tightly between the older layers. The dentin is covered by vitrodentin, an enamel-like substance produced by the ectoderm that is even harder than dentin. Once the dentin has broken through the epidermis, it can no longer be covered by enamel.