Its called sarcopterygians and they're categorized as a different class due to having different respiratory and circulatory systems.
Again, fish is not a class.
@anasawad
You don't seem to have taken on board the cladistic revolution in palaeontology from the 1990s onwards. The traditional classifications of the animal kingdom were fundamentally
arbitrary, being based mainly on the physical appearance of organisms. DNA analysis and new discoveries in the evolutionary lineages of these species has led to a revolution in the way we look at these things. A species, for example, is now seen as a
lineage, and not merely all the living organisms which happen to look the same. Fish, for example, are now defined as the vertebrates which appeared in the early Cambrian
and all of their descendants, which includes ourselves. We are therefore fish. Highly derived fish, but fish nonetheless. Hindsite is right about one thing: a fish can only give birth to another fish, a mammal can only give birth to another mammal, and a human can only give birth to another human. This means that we are fish, in exactly the same way that we are mammals and in exactly the same way that we are humans.
"Politics is the art of looking for trouble, finding it everywhere, diagnosing it incorrectly and applying the wrong remedies." - Marx (Groucho)