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Show 1876.] AND ON THE CLASSIFICATION OF FISHES. 57 In accordance with the view thus suggested, the humerus in the . . ***•<**' • • chiropterygium is the homologue of the proximal mesomere or joint of the axis of the archipterygium, while the radius and the ulna are the homologues of the proximal ends of praeaxial and postaxial parameres of the archipterygium. The confirmation or refutation of this hypothesis is to be sought in development, and in the condition of the limbs in those Palaeozoic Amphibia which may have more nearly approximated to Dipnoi than any existing or extinct forms at present known. I suggest it mainly in the hope of stimulating investigation in both these directions. IV. Taxonomy of Ceratodus, and Remarks on the Classification of Eishes. The indications afforded by the brain, the skull, and the limbs of Ceratodus are sufficient to show that it occupies a curiously central position among the Ichthyopsida, being allied on one side to the Amphibia, on another to the Chimseroidei and Plagiostomi, and on yet another to the Ganoidei-especially to that group of the Ganoids which I have termed Crossopteryyidee, and to the affinities of which with Ijepidosiren I called attention in 1861. But even Dipterus, which approaches Ceratodus and Lepidosiren so closely in its dentition and in the form of its fins, is far more similar to Polypterus and Amia in other respects ; and there is, at present, no reason to believe that any of the Crossopterygian Ganoids possessed other than a hyostylic skull, or differed from Polypterus in those respects in which Polypterus differs from the existing Dipnoi. All known Crossopterygians have jugular plates, of which there is no trace in the Dipnoi. And as to the position of the anterior nares, which appear to have been situated on the under face of the broad snout, not only in Dipterus, but in Osteolepis and Diplopterus, I have shown above that, so far from being a diagnostic character of the Dipnoi, it is simply an embryonic feature retained in them, the Selachians, and very probably in many of the early Ganoidei. On the other hand, in Amia, there is an even closer approximation between the Ganoids and the Teleosteans than can at present be shown to exist between any Ganoids and the Dipnoi; while the differences between the Dipnoi and the Chimaeroidei, and between the Chimae-roidei and the Plagiostomi respectively, are not less than those between the Ganoids and the Dipnoi. It seems to me, therefore, that by forming the Dipnoi, Ganoidei, Chimaeroidei, and Plagiostomi into a group of " Palaeichthyes," from which the Teleostei are excluded, as Dr. Gunther proposes to do, the differences between the Teleostei and the other hyostylic fishes are brought into undue prominence, and that it is better to retain the Miillerian groups of Dipnoi (Sirenoidei, Miiller), Ganoidei, Teleostei, Plagiostomi, and Chimaeroidei (Holocephali, Miiller) as equivalent and distinct natural assemblages. In discussing any system of classification, however, it must be |