diphy·cercal \|difē, -fə̇+\adjective Etymology:diphy- + -cercal 1.of a tail fin: having the upper and lower portions alike or nearly so and the vertebral column extending to the tip without any upturning < in lungfish and crossopterygians this symmetry is attained by the evolution of a diphycercal tail — A.S.Romer > 2.: having a diphycercal tail fin