2. _Ganoids._--With angular bony scales, covered with a thick stratum of enamel: to this order belong the fossil Lepidoides, Sauroides, Pycnodontes, and Coelacanthi; the recent Polypterus, Lepidosteus, Sclerodermi, Gymnodontes, Lophobranches, and Siluroides; also the Sturgeons. 3. _Ctenoids._--With rough scales, which have their free margins denticulated: Chætodontidæ, Pleuronectidæ, Percidæ, Polyacanthi, Sciænidæ, Sparidæ, Scorpænidæ, Aulostomi. 4. _Cycloids._--With smooth scales, the hind margin of which lacks denticulation: Labridæ, Mugilidæ, Scombridæ, Gadoidei, Gobiidæ, Murænidæ, Lucioidei, Salmonidæ, Clupeidæ, Cyprinidæ. We have no hesitation in affirming that if Agassiz had had an opportunity of acquiring a more extensive and intimate knowledge of existing fishes before his energies were absorbed in the study of their fossil remains, he himself would have recognised the artificial character of his classification. The distinctions between cycloid and ctenoid scales, between placoid and ganoid fishes are vague, and can hardly be maintained. As far as the living and post-cretacean forms are concerned, the vantage-ground gained by Cuvier was abandoned by him; and therefore his system could never supersede that of his predecessors, and finally shared the fate of every classification based on the modifications of one organ only. But Agassiz has the merit of having opened an immense new field of research by his study of the infinite variety of fossil forms. In his principal work, “Recherches sur les Poissons fossiles,” (Neuchatel, 1833–43, 4to, atlas in fol.), he placed them before the world arranged in a methodical manner, with excellent descriptions and illustrations. His power of discernment and penetration in determining even the most fragmentary remains is truly astonishing; and if his order of Ganoids is an assemblage of forms very different from that as it is circumscribed now, he was at any rate the first who recognised that such an order of fishes exists. * * * * * [Sidenote: J. Müller.] The discoverer of the _Ganoidei_ was succeeded by their explorer, JOHANNES MÜLLER (born 1801, died 1858). In his classical memoir “Ueber den Bau und die Grenzen der Ganoiden” (Berlin, 1846; 4to), he showed that the Ganoids differed from all the other osseous fishes, and agreed with the Plagiostomes, in the structure of their heart. By this primary character, all heterogeneous elements, as _Siluroids_, _Osteoglossidæ_, etc., were eliminated from the order as understood by Agassiz. On the other hand, he did not recognise the affinity of _Lepidosiren_ to the Ganoids, but established for it a distinct sub-class, _Dipnoi_, which he placed at the opposite end of the system. By his researches into the anatomy of the Lampreys and Amphioxus, their typical distinctness from other cartilaginous fishes was proved; they became the types of two other sub-classes, _Cyclostomi_ and _Leptocardii_.