Fish are broadly divided into three classes based on their diverse population. Osteichthyes or bony fish is a class of fish that includes common species such as salmon, tuna, and even eels. There are over 30,000 species in the Osteichthyes group. Chondrichthyes are also known as cartilaginous fish because they have jaws that belong to the Gnathostomata division. This category includes sharks, rays, and skates. Placoderms are a type of armored prehistoric fish that lived from the Silurian period to the end of the Devonian period. They are considered fossils. The head and neck of these fish were heavily armored with bone.