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.
the GNU version of the Collaborative International Dictionary of English
adjective.
Having a caudal fin extending around the end of the vertebral column, like that which is first formed in the embryo of fishes; diphycercal.