634 ICHTHYOLOGY [HISTORY AND he had failed to understand the very nature of cartilage, apparently comprising under this term any skeletal frame work of less firmness than ordinary bone. Hence he con sidered Lophius, Cydopterus, Synynathus to be cartilaginous fishes. Adopting the position and development of the ven tral fins as a highly important character, he had been obliged to associate fishes having rudimentary and inconspicuous ventral fins, like Trichiurus, Xiphias, &c., with the true eels. 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 Palceoniscus to those of the living Polypterus and Lepidos teus, the prolongation and identity of structure of the upper caudal lobe in Palceoniscus and the sturgeons, the presence of peculiar " fulcra " on the anterior margin of the dorsal fin in Palceoniscus and Lepidosteiis, and inferred from these facts that the fossil genus was allied either to the stur geons 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 sys tem a member of the order of Mcdacopterygii abdominales. Agassiz. It was left to L. Agassiz (1807-73) 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 Poly pterus 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. Plaeoids. Without scales proper, but with scales of enamel, sometimes largo, sometimes small, and reduced to mere points (Rays, Sharks, and -Cyclostomi, with the. fossil Hybodontes). 2. Ganoids. With angular bony scales, covered with a thick stratum of enamel : to this order belong the fossil Lepidoides, Sauroides, Pycnodontes, and Ccelacanthi ; the recent Polypterus, Lepidosteus, Sclerodermi, Gymuodontes, Lophobranches, and Siluroides ; also the Sturgeons. 3. Ctcnoids. With rough scales, which have their free margins denticulated : Chsetodontidse, Pleuroneetidse, Perjidse, Polyacanthi, Scisenidae, Sparidse, Scorpsenidse, Aulostomi. 4. Cyc loids. With smooth scales, the hind margin of which lacks denti- cnlation : Labridre, ihigilidse, Scombridte, Gadoidei, Gobiidre, Murrenidje, Lucioidei, Salmonidae, Clupeida% Cyprinidie. 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 fossil remains, ho would himself have recognized 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. So fir as the living and post- Cretacean forms are concerned, he abandoned the vantage- ground gained by Cuvier ; and therefore his system could never supersede that of his predecessor, 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 descrip tions 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 what is now understood by that term, he was at any rate the first who recognized that such an order of fishes exists. The discoverer of the Ganoidei was succeeded by their J. Mil explorer, Johannes Miiller (1801-58), In his classical memoir Ueber den 2>ctu und die Grenzen der Gunoidcn, Berl., 1846, he showed that the Ganoids differ from all the other osseous fishes, and agree with the Plagiostomes, in the structure of the heart. By this primary character, all heterogeneous elements as Siluroids, Onteoglossidw, etc., were eliminated from the order as understood by Agassiz. On the other hand, he did not recognize the affinity of Lepidosiren to the Ganoids, but established for it a distinct subclass, 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 subclasses, Cyclostomi and Lep/ocardii. Miiller proposed several other not unimportant modifica tions of the Cuvierian system ; and, although all cannot be maintained as the most natural arrangements, yet his researches have given us a much more complete knowledge of the organization of the Teleosteous fishes, and later inquiries have shown that, on the whole, the combinations proposed by him require only some further modification and another definition to render them perfectly natural. The discovery (in the year 1871) of a living representa tive of a genus hitherto believed to be long extinct, Cera- todus, threw a new light on the affinities of fishes. The writer of the present article, who had the good fortune to examine this fish, was enabled to show that, on the one hand, it was a form most closely allied to Lepidosiren, and, on the other, that it could not be separated from the Ganoid fishes, and therefore that Lepidosiren also was a Ganoid, a relation already indicated by Huxley in a previous paper on "Devonian Fishes." This discovery led to further considera tion 1 of the relative characters of M tiller s subclasses, and to the system which is followed in the present article. Having followed the development of the ichthyological system down to the most recent dak ; ru have to retrace our steps to enumerate the most important contributions to ichthyology which appeared contemporaneously with or sub sequently to the publication of the great work of Cuvier and Valenciennes. As in other branches of zoology, almost every year was marked by increased activity. For the sake of convenience we may arrange these works under three heads. I. VOYAGES, CONTAINING GENERAL ACCOUNTS OF ZOOLOGICAL Biblio COLLECTIONS. graph} A. French. 1 . Voyage autour du monde i ur les Corvettes de S. M. I Uranic et la PJiysicicnne, sous le commandemcnt de M. Frcycinet, "Zoologie Poissons," par Quoy et Gaimard, Paris, 1824. 2. Voyage de la Coquille, " Zoologie," par Lesson, Paris, 1826-30. 3. Voyage de V Astrolabe, sous le commandemcnt de M. J. Dumont d Urville, "Poissons," par Quoy et Gaimard, Paris, 1834. 4. Voyage au Pole Sud par M. J. Dumont d Urville, " Poissons," par Hombron et Jac- quinot, Paris, 1853-4. B. English. 1. Voyage of II. M.S. Sulphur, "Fishes," by J. Richardson, Loud., 1844-45. 2. Voyage of II.M.SS. Erebus and Terror, "Fishes," by J. Richardson, Lond., 1846. 3. Voyage of H. M.S. Beagle, "Fishes," by L. Jenyns, Lond., 1842. 4. Voyage of 11. M.S. Challenger, "Fishes," by A. Giinther. C. German. 1. Heine der osterrcichischen Freyatte Novara, "Fische, " von R. Kner, Vienna, 1865. II. FATJN/E. A. Great Britain.- 1. R. Parnell, The Natural History of the Fishes of the Firth of Forth, Edin., 1838. 2. W. Yarrell, A His tory of British Fishes, 3d edit., Lond., 1859. 3. J. Couch, History of the Fishes of the British Islands, Lond., 1862-65. ]>. Denmark and Scandinavia. 1. H. Krb yer, DaiimarttsFiske, Copenhagen, 1838-53. 2. S. Kilsson, Skandinavisk Fauna, vol. iv. " Fiskarna," Lund, 1855. 3. Fries och Ekstrom, Skandinamans Fiskar, Stockh., 1836. C. Russia. 1. Nordmann, "Ichthyologie Pontique," in Deuai- dolf s Voyage dans la liussie meridionalc, torn, iii., Paris, 1840. D. Germany. 1. Heckel und Kner, iJic Susswasser-fische der
1 " Description of Ceratodus," Phil. Trans., 1871, ii.
