_Abdominaux._ _Subbrachiens._ _Apodes._ --- --- --- Cyprinoïdes. Gadoïdes. Murénoïdes. Siluroïdes. Pleuronectes. Salmonoïdes. Discoboles. Clupéoïdes. Lucioïdes. 2. _A Mâchoire Supérieure Fixée._ Selérodermes. Gymnodontes. II. A BRANCHIES EN FORME DE HOUPPES. Lophobranches. B. CARTILAGINEUX OU CHONDROPTÉRYGIENS. Sturioniens. Plagiostomes. Cyclostomes. We have only to compare this system with that of Linnaeus if we wish to measure the gigantic stride made by ichthyology during the intervening period of seventy years. The various characters employed for classification have been examined throughout the whole class, and their relative importance has been duly weighed and understood. The important category of "family" appears now in Cuvier's system fully established as intermediate between genus and order. Important changes in Cuvier's system have been made and proposed by his successors, but in the main it is still that of the present day. Cuvier had extended his researches beyond the living forms, into the field of palaeontology; he was the first to observe the close resemblance of the scales of the fossil _Palaeoniscus_ to those of the living _Polypterus_ and _Lepidosteus_, the prolongation and identity of structure of the upper caudal lobe in _Palaeoniscus_ and the sturgeons, the presence of peculiar "fulcra" on the anterior margin of the dorsal fin in _Palaeoniscus_ and _Lepidosteus_, and inferred from these facts that the fossil genus was allied either to the sturgeons or to _Lepidosteus_. But it did not occur to him that there was a close relationship between those recent fishes. _Lepidosteus_ and, with it, the fossil genus remained in his system a member of the order of _Malacopterygii abdominales_. Agassiz. It was left to L. Agassiz (1807-1873) to point out the importance of the structure of the scales as a characteristic, and to open a path towards the knowledge of a whole new subclass of fishes, the _Ganoidei_. Impressed with the fact that the peculiar scales of _Polypterus_ and _Lepidosteus_ are common to all fossil osseous fishes down to the Chalk, he takes the structure of the scales generally as the base for an ichthyological system, and distinguishes four orders:--