Fish are generally covered in scales and have fins and a propulsive tail. There are several classes, Agnatha without jaws, the extinct Placodermi, modern Osteichthyes with a bone skeleton and Chondrichthyes with a cartilage skeleton.

Sharks are Chondrichthyes and have very large numbers of teeth which are constantly being lost and replaced. The example of a shark's tooth shown in the picture is Otodus obliquus from the Eocene of Sheppey, Kent, England.