[CHAPTER XV.]
FOSSIL ICHTHYOLOGY; COMPRISING THE GANOID, CTENOID, AND CYCLOID FISHES.
The fishes we have hitherto examined belong to the first order, the Placoidians; we now pass to the fossil remains of the second order, the Ganoidians, which are distinguished by their brilliant angular scales, formed of osseous or horny plates, densely covered with enamel. This order contains six or more families, comprising many genera and numerous species; our investigation must be restricted to a selection from the principal genera of the Ganoids, properly so called, and of the Sauroids, or lizard-like fishes.[530]
[530] The fishes of these orders are described in Poiss. Foss. tom. ii.
The first family, termed Lepidoides, contains several genera, which are defined as possessing either numerous rows of brush-teeth, or of obtuse conical teeth; flat, rhomboidal scales, arranged parallel with the body; and an osseous, or partially osseous, skeleton. In one division of this family, the body is either elongated or fusiform, the mouth furnished with brash-teeth only, and the tail heterocercal, or unequally bilobed (see [p. 576]). To this group belong several genera, which are restricted to the Secondary formations more ancient than the Oolite; while the other group, with homocercal tails, lived in the Oolitic and Cretaceous seas. Two genera, in particular, abound in the Permian and Carboniferous strata; namely, Amblypterus and Palæoniscus.[531]
[531] For the characters, affinities, and distribution of these and the allied genera of the Heterocerque Ganoid fishes, see Sir P. Egerton’s Memoir in the sixth volume of the Quarterly Geological Journal.
Amblypterus. [Lign. 187]. Wond. p. 740. Bd. pl. xxvii.b.—The fishes of this genus, as the name indicates, have very large and wide fins, composed of numerous rays. The scales are rhomboidal and finely enamelled; the tail is heterocercal. The figures referred to convey a correct idea of the form and external characters. Beautiful pyritous imprints of Amblypteri occur in the Carboniferous slate of Saarbrück, in Lorraine; and fine specimens in the ironstone nodules of the same locality. On the shore at Newhaven, near Leith, similar fossils occur in nodules washed out of the cliffs of coal-shale (Bd. p. 278).
Palæoniscus. Ly. p. 304.—The fishes of this genus differ from those of Amblypterus in the relatively moderate size of the scales, and the numerous little rays on their margins. They have rhomboidal scales, which in some species are very small, and in others large. They have numerous brush-teeth. Several peculiar species, found in the marl-slates and magnesian limestones of the Permian system, are very widely distributed, occurring in the British Isles, Germany, and the United States.
In some localities the small species occur in groups; on a slab of red sandstone, in the Museum of the Geological Society, from Tyrone, between two and three hundred perfect fishes (P. catopterus) are imbedded on a space not exceeding two feet square.
A remarkable circumstance relating to the fishes of this genus is the almost constant absence of the bodies of the vertebræ in otherwise well-preserved specimens, and in which the spinal processes and the ribs are entire: occasionally, however, examples occur with some of the vertebræ perfect. An explanation of the above phenomenon may perhaps be found in the probable originally cartilaginous nature of the bodies of the vertebræ, and the osseous structure of the enduring apophyses and ribs;[532] while those rare specimens which possess a few bony vertebræ may be regarded as exceptions, in which ossification took place in a structure essentially cartilaginous.