MAGNESIAN LIMESTONE AND MARL SLATE. 
389 
it is thirty feet thick, this slate has yielded many fine speci¬ 
mens of fossil fish—of the genera Palmojiiscus ten species, 
Pygopterus two species, Coelacanthus two species, and Pla- 
tysomus two species, which as genera are common to the old¬ 
er Carboniferous formation, but the Permian species are pe¬ 
culiar, and, for the most part, identical with those found in 
the marl-slate or copper-slate of Thuringia. 
Fig. 41T. 
Restored outline of a fish of the genus Palceoniscus, Agass. 
Palceothrissum, Blainville. 
The Palceoniscus above mentioned belongs to that division 
of fishes w^hich 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 
Fig. 418. Fig. 419. 
Shark. Heterocercal. Shad. {Clu2Dea. Herring tribe.) 
Homocercal. 
caudal lobe. (See Fig. 418.) The “ Homocercal ” fish, which 
comprise almost all the 9000 species at present known in the liv¬ 
ing 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. 419.) Now it is a singular fact, 
first pointed out by Agassiz, that the heterocercal form, which 
is confined to a small number of genera in the existing crea¬ 
tion, is universal in the magnesian limestone, and all the more 
ancient formations. It characterizes the earlier periods of 
the earth’s history, whereas in the secondary strata, or those 
newer than the Permian, the homocercal tail predominates. 
A full description has been given by Sir Philip Egerton of 
the species of fish characteristic of the marl-slate, in Prof. 
King’s monograph before referred to, where figures of the 
