This so called heterocercal tail is only known to exist, as a permanent adult feature, in the sturgeons of to-day.    From Wordnik.com.  [Louis Agassiz: His Life and Correspondence] Reference

Paddlefish are not closely related to sharks, but they do share some common characteristics including a skeleton primarily composed of cartilage, and a deeply forked, abbreviate heterocercal tail fin (the top fin lobe is slightly larger than the lower fin lobe).    From Wordnik.com.  [Archive 2007-01-01] Reference

Their tails, too, though both heterocercal, were diverse in their type.    From Wordnik.com.  [The Cruise of the Betsey or, A Summer Ramble Among the Fossiliferous Deposits of the Hebrides. With Rambles of a Geologist or, Ten Thousand Miles Over the Fossiliferous Deposits of Scotland] Reference

These are the even-lobed, or homocercal (Fig. 68), and the uneven-lobed, or heterocercal (Fig. 69).    From Wordnik.com.  [Darwin, and After Darwin (Vol. 1 and 3, of 3) An Exposition of the Darwinian Theory and a Discussion of Post-Darwinian Questions] Reference

Now, in the development of a teleost fish (Fig. 68), as has been shown by Alexander Agassiz, the tail-fin is first like Fig. 70; then becomes heterocercal, like Fig. 69; and, finally, becomes homocercal like Fig. 68.    From Wordnik.com.  [Darwin, and After Darwin (Vol. 1 and 3, of 3) An Exposition of the Darwinian Theory and a Discussion of Post-Darwinian Questions] Reference

The lecturer dwelt chiefly on the argument of Agassiz, which he summarises as follows: "Homocercal fishes have in their embryonic state heterocercal tails; therefore heterocercality is, so far, a mark of an embryonic state as compared with homocercality, and the earlier heterocercal fish are embryonic as compared with the later homocercal.".    From Wordnik.com.  [More Letters of Charles Darwin — Volume 1] Reference