designating, of, or having a tail fin in which the upper lobe is larger than the lower and contains the upturned end of the spinal column, as in certain fishes, esp. sharks
See heterocercal in American Heritage Dictionary 4
(hĕtˌə-rō-sûrˈkəl)
adjective
Relating to, having, or being a tail fin in which the upper lobe is larger than the lower and the vertebral column extends into the upper lobe, as in sharks.