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fusion of life met with in the Silurian and Devonian strata presupposes an immeasurably long antecedent period during which life had already existed and gradually increased to the multitudes of the Silurian era. We discover in it but scanty remains of marine plants, and only marine animals; but these are so heterogeneous and varied in form, that they alone would oblige us to infer the existence of coasts, shallow or deep oceanic regions, and a number of geographical conditions on which we see the variety and extent of animal life to be dependent. Besides numerous forms of corals more nearly allied to still existing families, we find the quite peculiar group of Graptolites (fig. 9), which, although not actual polypes, might be ranged next to the so-called Medusapolypes, and thus justify the inference that preparation was being made for the appearance of the higher forms of the Cœlenterata, the Medusa.
The Articulata are represented by the Trilobites (fig. 10, Trilobites remipes), a crab-like form which recalls the present group of the Lamellibranchiata, but has not hitherto admitted of any closer definition, as in none of the many thousand specimens examined, of the forms (about 2000) known in the Silurian and Devonian strata, have the legs been preserved. In these three-lobed crabs, the head, trunk, and tail distinctly appear, as well as the threefold transverse division. The two composite eyes already indicate a high grade of organization. The power of rolling themselves up,
which they have in common with several of the crabs now inhabiting shallow waters and coasts, and likewise their general habit, allow us to infer that they also were denizens of coasts.
The Molluscs were mainly represented by Brachiopoda and Cephalopoda. However, as Bivalves and Gaster
opoda were also in existence, the appearance of this, the most ancient molluscous fauna known, differs from the present one only in its numerical proportions, and in the
circumstance, certainly very important, that of the Cephalopoda the Nautilus alone is found. The Brachiopoda soon attain to their highest development, and have lingered on till now in a greatly reduced state. Among the Conchifera, the Dimyariæ take the lead in the course of the later period; and with regard to the Gasteropods, we will merely observe that they constantly increase in internal complexity and variety as they approach more recent periods, and that the terrestrial and fresh-water species are occasionally found in the carboniferous formation, though in number and variety they belong primarily to the Tertiary era. To the Cephalopoda we must return again. Of the Vertebrata in the Silurian strata we know only the remains of peculiar Fishes whose kindred must be sought among the sharks and rays.
In the period of the Devonian or upper Transition rocks, the surface of the earth had assumed, at least in places, a more smiling appearance. Here begins the first record of terrestrial plants. As to the character of the fauna, the rapid decrease of the Trilobites is worthy of notice, and the appearance of the important genus of the Cephalopoda, Clymenia, subsequently replaced by the Ammonites. Above all, we must note the increased abundance of fish which still form the sole representatives of the Vertebrata, and held undisputed sway in the seas of that period. Besides the sharks, there are the mailed Ganoids. It is true, the fish, the hinder part of which is here portrayed (Fig. 11, Palæoniscus), belongs only to the upper Coal and Zechstein formation; but it is necessary even now to point out the characteristics of the true Ganoids which floundered about the Silurian seas in somewhat extraordinary forms. Agassiz
terms them Placoids, from the rhombic scales, provided with a layer of enamel highly favourable to preservation, and covering the whole surface in oblique rows.
The vertebral column, as in the sharks, enters the upper flap of the tail and renders it strikingly unsymmetrical. The Ganoids are, as comparative anatomy has proved with certainty, a development of the shark-like fishes, if not decidedly of a higher grade. The Ganoids, therefore, presuppose the shark.
The carboniferous period owes its name to the enormous accumulation occurring in its midst, of the remains of terrestrial plants, fern-like Calamites, and more especially of Sigillaria and Lepidodendra, standing between vascular Cryptogams and Conifers. They formed tropical bog-forests, such as Franz Unger some years ago attempted to restore in an ingenious composition. In these steaming primæval forests, differing from the early beginnings of antecedent periods by their extent and luxuriance, new phases of animal life become manifest-scorpions, myriapods, and insects in other words, air-breathing Articulata, and likewise the first air-breathing Vertebrata. The latter,
the Cheirotheria, or Labyrinthodonta (colossal Batrachians) possess pre-eminently amphibian characters, and exhibit, for example, several important characteristics of the Batrachian skull, whereas their skin-covering recalls the scale-armour of the Saurians. Thus we find characters combined which are subsequently divided among different groups. There are also traces of huge sea-lizards. But here, and likewise in the magnesian limestone formation, these amphibian-like animals still keep in the background amid the profusion of Ganoids, which especially characterizes some of the strata of the magnesian limestone formation, the Kupferschiefer, or cupriferous marl formation. For the sake of classification, the Zechstein is not unfitly supposed to conclude a great period of organic development: the series of formations from the Silurian to the end of the Zechstein is termed palæazoic; and those which follow, the Trias, Oolite, and Cretaceous formations, are summed up as mesozoic.
The Trilobites, the mailed Ganoids, and others have now disappeared, and the enormous development of reptile life stamps this middle period. The Trias as yet possesses no true Teleostei. The Labyrinthodonta still predominate; while the Archæosauros and the Proterosaurus, which had already appeared in the Dyas, are replaced by more numerous forms approximating to the true reptiles. One single discovery in the upper member of the Trias-the teeth of a predatory marsupial-has supplied us with the most ancient traces of a mammal. It might be inferred, even from the petrographic character of the oolitic strata, that this era must have been, on the whole, far more favourable to the development of animal life than the more perturbed Triassic period, or