Thanks to the researches of Agassiz, and especially Sir P. Egerton, the ichthyological fauna of the Lias is, perhaps, the best known of the Mesozoic era, 152 species having been described. Of the various localities, Lyme Regis has yielded more than any other, nearly all the Liassic genera being represented there by not less than seventy-nine species. The Hybodonts and Cestracionts continue in their fullest development. Holocephales (Ischyodus), true Sharks (Palæoscyllium), Rays (Squaloraja, Arthropterus), and Sturgeons (Chondrosteus) make their first appearance; but they are sufficiently distinct from living types to be classed in separate genera, or even families. The Ganoids, especially Lepidosteoids, predominate over all the other fishes: Lepidotus, Semionotus, Pholidophorus, Pachycormus, Eugnathus, Tetragonolepis, are represented by numerous species; other remarkable genera are Aspidorhynchus, Belonostomus, Saurostomus, Sauropsis, Thrissonotus, Conodus, Ptycholepis, Endactis, Centrolepis, Legnonotus, Oxygnathus, Heterolepidotus, Isocolum, Osteorhachis, Mesodon. These genera offer evidence of a great change since the preceding period, the majority not being represented in older strata, whilst, on the other hand, many are continued into the succeeding oolithic formations. The homocercal termination of the vertebral column commences to supersede the heterocercal, and many of the genera have well ossified and distinctly segmented spinal columns. Also the cycloid form of scales becomes more common: one genus (Leptolepis) being, with regard to the preserved hard portions of its organisation, so similar to the Teleosteous type that some Palæontologists refer it (with much reason) to that sub-class.
[See E. Sauvage, Essai sur la Faune Ichthyologique de la période Liasique. In “Bibl. de l’école des hautes études,” xiii. art. 5. Paris 1875. 8o.]
As already mentioned, the Oolithic formations show a great similarity of their fish-fauna to that of the Lias; but still more apparent is its approach to the existing fauna. Teeth have been found which cannot even generically be distinguished from Notidanus. The Rays are represented by genera like Spathobatis, Belemnobatis, Thaumas; the Holocephali are more numerous than in the Lias (Ischyodus, Ganodus). The most common Ganoid genera are Caturus, Pycnodus, Pholidophorus, Lepidotus, Leptolepis, all of which had been more or less fully represented in the Lias. Also Ceratodus is continued into it.
The Cretaceous group offers clear evidence of the further advance towards the existing fauna. Teeth of Sharks of existing genera Carcharias (Corax), Scyllium, Notidanus, and Galeocerdo, are common in some of the marine strata, whilst Hybodonts and Cestracionts are represented by a small number of species only; of the latter one new genus, Ptychodus, appears and disappears. A very characteristic Ganoid genus, Macropoma, comprises homocercal fishes with rounded ganoid scales sculptured externally and pierced by prominent mucous tubes. Caturus becomes extinct. Teeth and scales of Lepidotus (with Sphærodus as subgenus), clearly a freshwater fish, are widely distributed in the Wealden, and finally disappear in the chalk; its body was covered with large rhomboidal ganoid scales. Gyrodus and Aspidorhynchus occur in the beds of Voirons, Coelodus and Amiopsis (allied to Amra), in those of Comen, in Istria. But the Palæichthyes are now in the minority; undoubted Teleosteans have appeared, for the first time, on the stage of life in numerous genera, many of which are identical with still existing fishes. The majority are Acanthopterygians, but Physostomes and Plectognaths are likewise well represented, most of them being marine. Of Acanthopterygian families the first to appear are the Berycidæ, represented by several very distinct genera: Beryx; Pseudoberyx with abdominal ventral fins; Berycopsis with cycloid scales; Homonotus, Stenostoma, Sphenocephalus, Acanus, Hoplopteryx, Platycornus with granular scales; Podocys with a dorsal extending to the neck; Acrogaster, Macrolepis, Rhacolepis from the chalk of Brazil. The position of Pycnosterynx is uncertain, it approaches certain Pharyngognaths. True Percidæ are absent, whilst the Carangidæ, Sphyrænidæ, Cataphracti, Gobiidæ, Cottidæ, and Sparidæ are represented by one or more genera. Somewhat less diversified are the Physostomes, which belong principally to the Clupeidæ and Dercetidæ, most of the genera being extinct; Clupea is abundant in some localities. Scopelidæ (Hemisaurida and Saurocephalus) occur in the chalk of Comen in Istria, and of Mæstricht. Of all cretaceous deposits none surpass those of the Lebanon with regard to the number of genera, species, and individuals; the forms are exclusively marine, and the remains in the most perfect condition.
In the Tertiary epoch the Teleosteans have almost entirely replaced the Ganoids; a few species only of the latter make their appearance, and they belong to existing genera, or, at least, very closely allied forms (Lepidosteus, Amia, Hypamia, Acipenser). The Chondropterygians merge more and more into recent forms; Holocephali continue, and still are better represented than in the present fauna. The Teleosteans show even in the Eocene a large proportion of existing genera, and the fauna of some localities of the Miocene (Oeningen) is almost wholly composed of them. On the whole, hitherto more than one-half have been found to belong to existing genera, and there is no doubt that the number of seemingly distinct extinct genera will be lessened as the fossils will be examined with a better knowledge of the living forms. The distribution of the fishes differed widely from that of our period, many of our tropical genera occurring in localities which are now included within our temperate zone, and being mixed with others, which nowadays are restricted to a colder climate: a mixture which continues throughout the Pliocene.
A few families of fishes, like the freshwater Salmonidæ, seem to have put in their appearance in Post-pliocene times; however, not much attention has been paid to fish-remains of these deposits; and such as have been incidentally examined offer evidence of the fact that the distribution of fishes has not undergone any further essential change down to the present period.
[See E. Sauvage, Mémoire sur la Faune Ichthyologique de la période Tertiaire. Paris 1873. 8°.]