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Shark. Shad. Herring tribe. The Palaeoniscus 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.

It is likewise to be observed, that the fishes of this age, to the genera of which the names palaeoniscus, catopterus, platysomus, &c., have been applied, vanish, and henceforth appear no more.

At East Thickley, in Durham, where it is thirty feet thick, this slate has yielded many fine specimens of fossil fish of the genera Palaeoniscus ten species, Pygopterus two species, Coelacanthus two species, and Platysomus two species, which as genera are common to the older Carboniferous formation, but the Permian species are peculiar, and, for the most part, identical with those found in the marl-slate or copper-slate of Thuringia.

They are often scattered through the beds singly, and may be useful to a geologist in determining the age of the rock. Palaeoniscus comptus, Agassiz. Scale, magnified. Palaeoniscus elegans, Sedgwick. Under surface of scale, magnified. Palaeoniscus glaphyrus, Agassiz. Under surface of scale, magnified. Coelacanthus granulatus, Agassiz. Granulated surface of scale, magnified.

Some of this same tribe of shells, such as Camarophoria, allied to Rhynchonella, Spiriferina, and two species of Lingula, are specifically the same as fossils of the carboniferous rocks. Restored outline of a fish of the genus Palaeoniscus, Agassiz. Beneath the limestone lies a formation termed the marl-stone, which consists of hard calcareous shales, marl-slate, and thin-bedded limestones.

The chief other fishes of the coal era are named palaeothrissum, palaeoniscus, diperdus. Coal strata are nearly confined to the group termed the carboniferous formation. Thin beds are not unknown afterwards, but they occur only as a rare exception.