_B._ CARTILAGINEUX OU CHONDROPTÉRYGIENS. Sturioniens. Plagiostomes. Cyclostomes. We have to compare this system with that of Linnæus if we wish to measure the gigantic stride Ichthyology has made 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. Though Linnæus had formed a category of “Amphibia nantes” for fishes with a cartilaginous skeleton, which should coincide with Cuvier’s “Poissons Cartilagineux,” he had failed to understand the very nature of cartilage, apparently comprising by this term any skeletal framework of less firmity than ordinary bone. Hence he considered _Lophius_, _Cyclopterus_, _Syngnathus_ to be cartilaginous fishes. Adopting the position and development of the ventral fins as a highly important character, he was obliged to associate fishes with rudimentary and inconspicuous ventral fins, like _Trichiurus_, _Xiphias_, etc., with the true Eels. The important category of a “family” appears now in Cuvier’s system fully established as that 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 palæontology; he was the first to observe the close resemblance of the scales of the fossil _Palæoniscus_ to those of the living _Polypterus_ and _Lepidosteus_, the prolongation and identity of structure of the upper caudal lobe in _Palæoniscus_ and the Sturgeons, the presence of peculiar “fulcra” on the anterior margin of the dorsal fin in _Palæoniscus_ and _Lepidosteus_: inferring from these facts that that 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_. It was left to L. AGASSIZ (born 1807, died 1873) to point out the importance of the character of the structure of the scales, and to open a path towards the knowledge of a whole new sub-class 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:-- 1. _Placoids._--Without scales proper, but with scales of enamel, sometimes large, sometimes small and reduced to mere points (Rays, Sharks, and Cyclostomi, with the fossil _Hybodontes_).