The Collaborative International Dictionary
Protocercal
Protocercal \Pro`to*cer"cal\, a. [Proto- + Gr. ? the tail.] (Zo["o]l.) Having a caudal fin extending around the end of the vertebral column, like that which is first formed in the embryo of fishes; diphycercal.
Wiktionary
protocercal
a. (context zoology English) Having a caudal fin extending around the end of the vertebral column, like that which is first formed in the embryo of fishes; diphycercal.