Definition of Holocephali
: a subclass of Chondrichthyes that is sometimes made a separate class, includes the recent chimaeras and certain chiefly extinct related fishes some of which date from Devonian time, and is distinguished by a cartilaginous skeleton, gill clefts covered by a fold of skin, high compressed head with small narrow mouth and the dentition reduced to broad flat plates, and a body tapering off into a long tail
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