A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

Restored outline of a fish of the genus Palæoniscus, Agass. Palæothrissum, Blainville.

The Palæoniscus above mentioned belongs to that division of fishes which M. Agassiz has called "Heterocercal," which have their tails unequally bilobate, like the recent shark and sturgeon, and the vertebral column running along the upper caudal lobe. (See [fig. 340.]) The "Homocercal" fish, which comprise almost all the 8000 species at present known in the living creation, have the tail-fin either single or equally divided; and the vertebral column stops short, and is not prolonged into either lobe. (See [fig. 341.])

Fig. 340.

Shark.

Heterocercal.

Fig. 341.