Holocephali is a subclass of cartilaginous fish that includes species commonly known as chimeras. These fish are characterized by their unique features, such as a single gill cover and a long, slender body. They primarily inhabit deep-sea environments and are known for their distinctive flat, rounded heads. Chimeras possess a diet that mainly consists of crustaceans and mollusks, which they capture using their specialized teeth. Unlike their relatives in the subclass Elasmobranchii, which includes sharks and rays, chimeras have a more primitive evolutionary history, dating back over 400 million years.