Dictionary of Ichthyology  

isocercal = diphycercal (an internally and externally symmetrical tail fin. May be secondarily acquired from the homocercal condition by loss of the real caudal fin and the gaining of a new one from dorsal and anal elements, e.g. in Gadidae).

Tail of a hake, Merluccius productus, from "A Guide to the Study of Fishes" by D. S. Jordan, 1905. Photograph by Brian W. Coad.

Canadian Museum of Nature
www.briancoad.com