Synopsis Fish are classified in several classes by which its study becomes easier. Fish having similar properties are: contained in the same group. Various classes of fish are Thelodonti, Anaspida, Cephalaspidomorphi, Galeaspida, Pituriaspida, Osteostracy, Gnathostomata, Placodermi. Jewles fish are classified under the group Theolodonti, which are very similar to Heterostraci and are not armoured. The Pituriaspida are a small group of armoured jawless fish with tremendous nose-like rostrums, which live in the marine, deltaic environments of Middle Devonian Australia. Gnathstomata is the group of vertebrates with jaws. This group is in superclass, including the familiar classes of fish, birds, mammals and so forth and a sister group of jawless vertebrates Agnatha. The placodermi are armoured prehistoric fish known from fossils, dating from the Late Silurian to the end of the Devonian Periods. This eminent work has an exhaustive and exclusive coverage of fish in a simple, but effective style. This work would be capable to prove beneficial for all classes of readers.