protocercal

from The Century Dictionary.

• Having a primitive tail-fin: noting the embryonic stage of the vertical fins and tail of a fish, when these consist of a continuous skinfold along both upper and under sides of the body and around its tail-end.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Zoöl.) Having a caudal fin extending around the end of the vertebral column, like that which is first formed in the embryo of fishes; diphycercal.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective zoology Having a caudal fin extending around the end of the vertebral column, like that which is first formed in the embryo of fishes; diphycercal.