Dictionary of Ichthyology
heterocercal = the type of tail fin in which the vertebral column turns upwards into the upper lobe which is longer than the lower. Asymmetrical externally as well as internally, e.g. Elasmobranchii, Acipenseridae, Polyodontidae and in many larval Teleostomi. May also be used for an asymmetrical caudal fin but with epicercal defining upper lobe longer and hypocercal the lower lobe longer.