heterocercy

from The Century Dictionary.

• noun The state or quality of being heterocercal; inequality of the lobes of the caudal fin: opposed to homocercy.

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

• noun (Anat.) Unequal development of the tail lobes of fishes; the possession of a heterocercal tail.

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

• noun anatomy unequal development of the tail lobes of fishes; the possession of a heterocercal tail.