Heterocercal ,.All palaeozoic and most mesozoic fishes had a vertebrated tail, the vertebra) extending to its extremity, instead of stopping short at its commencement as in almost all existing fishes. As the vertebral column extended into the upper lobe of the tail, the two lobes were unequal, sometimes very much so; hence such were called by Agassiz heterocercal or une-ventailed fishes, while those of the ordinary form, the even-tailed, were called homocer-cal. The placoids (selachians or sharks) and the ganoids (sturgeons and gar pike), both the extinet and living species, had heterocercal tails; while ordinary fishes (etenoids and cycloids), from the oolite to the present time, have the lobes of the tail nearly or quite equal. This has been regarded as a mark of the sau-roid character of the early fishes; with other peculiarities, it indicates the high position of these ancient but seemingly prophetic types, and is among the facts that are difficult to explain on the Darwinian theory of development.

1. Heterocercal (Shark).

2. Homocercal (Salmon).