Real Dictionary

diphycercal in American English
designating, of, or having a tail fin in which the upper and lower lobes taper symmetrically to a point to which the spinal column extends
adjective: having a tail or caudal fin with the spinal column extending horizontally to the end of the tail, characteristic of lungfish, several other primitive fishes, and the juvenile stage of modern bony fishes

diphycercal in British English
adjective: of or possessing a symmetrical or pointed tail with the vertebral column extending to the tip, as in primitive fishes