The Swiss naturalist, accordingly, in adopting a new principle of classification, so essential in the case of the fossils of the old red sandstone, has made the scales and external organs the groundwork of his system. The classification of Cuvier and the older naturalists proceeds mainly upon the character and disposition of the fins. Hence the order of the Acanthopterygii, or thorny-finned; and the Malacopterygii, or the soft finned order.—The classification of M. Agassiz, proceeding upon the characters of the scales and plates, has given rise to the following orders, namely, the Placoid, or broad-plated scale; the Ganoid, or the shining-scale; the Ctenoid, or comb-shaped scale; and the Cycloid, or marginated scale. Upon the simple basis of these four orders, he has constructed his system and composed his “Poissons Fossiles,” the standard authority in fossil ichthyology, and elaborate monument of his learning and genius. The relations, as well as distinguishing peculiarities, of the fishes of the old red, are thus described by Agassiz:—“Of the Placoidian order,” he says, “the genera ctenacanthus, onchus, ctenoptychius, and ptychacanthus, are provided with spinous rays to the dorsal fins, resembling the gigantic ichthyodorulites of the carboniferous and jurassic formations, but differing in their less considerable size; they are distinguished among themselves by the forms and ornaments of their rays. In the order of ganoid fishes, the genera acanthodes, diplacanthus, cheiracanthus, and cheirolepis present themselves at first sight as a separate group; for although covered, like the others, with enameled scales, these are so small, that they impart to the skin the appearance of shagreen. The manner in which the fins are sustained by spinous rays, or the absence of these rays, and the position of the fins themselves, have served as characters in the establishing of these genera. The genera pterichthys, coccosteus, and cephalaspis, form a second group exceedingly curious: the considerable development of the head, its size, large plates which cover it, and which likewise extend over the greater portion of the trunk, and the movable appendages in the form of a wing, placed on the side of the head, give to them the most remarkable appearance. It is these peculiarities, indeed, which caused the class to which these genera belong for a long time to be misunderstood. The large bony and granulated plates of coccosteus, led to their being considered as belonging to trionyx: and it will be sufficient excuse for this error to call to recollection, that the greatest anatomist of our age had sanctioned this approximation. The form of the disc of the head of the cephalaspides, which has the appearance of a large crescent, and their more numerous, but very elevated scales, resembling the transverse articulations of the body, explain how it was possible to see in these fishes the trilobites of a particular genus. Lastly, the winged appendages of the sides of