The Collaborative International Dictionary of English v.0.48:

Protocercal \Pro`to*cer"cal\, a. [Proto- + Gr. ? the tail.]
   (Zool.)
   Having a caudal fin extending around the end of the vertebral
   column, like that which is first formed in the embryo of
   fishes; diphycercal.
   [1913 Webster]