The passage from the Palæozoic into the _Mesozoic_ era is not indicated by any marked change as far as fishes are concerned. The more remarkable forms of the Trias are Shark-like fishes represented by ichthyodorulithes like _Nemacanthus_, _Liacanthus_, and _Hybodus_; and Cestracionts represented by species of _Acrodus_ and _Strophodus_. Of the Ganoid genera _Cœlacanthus_, _Amblypterus_ (_Palæoniscidæ_), _Saurichthys_ persist from the Carboniferous epoch. _Ceratodus_ appears for the first time (Muschel-Kalk of Germany). 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. 8^o.]