. Elements of geology : a text-book for colleges and for the general reader. Geology. 34:2 PALAEOZOIC SYSTEM OF ROCKS. as the Ceratodus, Fig. 438, and the structure resembling a leg is shown in Figs. 447 and 448. 6. The teeth of many Devonian Ganoids are fluted or channeled on the outer surface near the base (Fig. 449, a). On section it is found that the inner surface next the pulp is deeply folded (Fig. 449, b). This is called labyrinthine structure. It is still more marked in early A mphibians, and may be regarded as an amphibian Fig. 449.—Structure of a Ganoid Tooth character. (after Agassi
. Elements of geology : a text-book for colleges and for the general reader. Geology. 34:2 PALAEOZOIC SYSTEM OF ROCKS. as the Ceratodus, Fig. 438, and the structure resembling a leg is shown in Figs. 447 and 448. 6. The teeth of many Devonian Ganoids are fluted or channeled on the outer surface near the base (Fig. 449, a). On section it is found that the inner surface next the pulp is deeply folded (Fig. 449, b). This is called labyrinthine structure. It is still more marked in early A mphibians, and may be regarded as an amphibian Fig. 449.—Structure of a Ganoid Tooth character. (after Agassiz). a. External form, nat- n t\ • tvi • n -n ri urai size; b, enlarged section, showing 7. Devonian Placoids were all Ces- stmcture. tracionts, i. e., they all had cobblestone- pavement teeth, instead of the lancet-shaped teeth characteristic of modern sharks. Devonian Fishes were Generalized Types.—Teleosts are typical fishes; Ganoids and Placoids, especially Devonian Ganoids and Pla- coids, were both connecting and embryonic types—i. e., along with their distinctive fish-characters they combined others which connect them with higher vertebrates, especially amphibians, and still others which are found in the embryos of Teleosts. The most important connecting characters of Ganoids, especially Devonian Ganoids, are: 1. An ex- ternal protective armor of thick, bony plates or scales, such as were possessed by early amphibians, and by many reptiles of the present time. 2. Large conical teeth channeled at the base, and of labyrinthine structure on section. This structure was very marked in early am- phibians. 3. A cellular air-bladder, freely supplied with blood, open- ing into the throat, and capable of being used to some extent as a lung. We do not know that this was true of Devonian Ganoids, but it prob- ably was, since it is true of all their nearest living allies, viz., Lepi- dosteus, Polypterus, Amia, and especially Ceratodus and Lepidosiren. 4. In many cases paired
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