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. 

A = diphycercal in Polypterus, B = heterocercal in Acipenser, C = homocercal in Amia and D = homocercal in Salmo from Kingsley's "Textbook of Vertebrate Zoology, 1906. Photograph by Brian W. Coad.

Canadian Museum of Nature