The Collaborative International Dictionary of English v.0.48:

Heterocercal \Het`er*o*cer"cal\, a. [Hetero- + Gr. ? tail.]
   (Anat.)
   Having the vertebral column evidently continued into the
   upper lobe of the tail, which is usually longer than the
   lower one, as in sharks.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heterocercal
    adj 1: possessing a tail with the upper lobe larger than the
           lower and with the vertebral column prolonged into the
           upper lobe [ant: homocercal]