During the incalculable ages which the formation of the various systems of secondary strata must have comprised, we find no evidence in the fossils hitherto observed, of the existence of birds and mammalia, as the characteristic types of the faunas of the dry land. On the contrary, throughout the immense accumulations of the spoils of the ancient islands and continents, amidst innumerable relics of reptiles of various orders and genera, a few jaws and bones of two or three kinds of extremely small marsupials, and the bones of a species of wader, are the sole indications of the presence of the two grand classes of Aves and Mammalia, which now constitute the chief features of the terrestrial zoology of almost all countries. The earliest indications of air-breathing vertebrata in the ancient secondary formations are those of small saurian reptiles in the carboniferous strata; a few vestiges occur in the succeeding group, the Permian. In the next epoch, the Triassic, colossal Batrachians (_Labyrinthodonts_) appear; and on some of the strata of this formation are the footmarks of numerous bipeds, presumed to be those of birds; but at present the evidence of the bones of the animals that made those imprints is required to establish the hypothesis. In the succeeding eras, the Lias, Oolite, Wealden, and Cretaceous, swarms of reptiles of numerous genera and species everywhere prevail; reptiles fitted to fly through the air, to roam over the land, to inhabit the lakes, rivers, and seas; and yet not one identical with any existing forms! These beings gradually decline in numbers and species as we approach the close of the secondary periods, and are immediately succeeded in the eocene epoch, by as great a preponderance of warm-blooded vertebrata--birds and mammalia--as exists at the present time; and an equal decadence in the class of reptiles. With the Cretaceous Formation the "Age of Reptiles" may be said to terminate. XVII.--Fossil Reptiles of the Wealden. _The Iguanodon._ The fluviatile deposits (termed _Wealden_), which in the south-east of England, and in the north of Germany, are intercalated between the oolitic and cretaceous formations, abound in the bones of terrestrial, fresh-water, and marine reptiles, comprising some of the most colossal land-saurians which have hitherto been brought to light. These remains belong to various genera of Chelonians, Saurians, and Crocodilians; and with these are associated those of flying lizards (_Pterodactyles_), Plesiosauri, gigantic whale-like reptiles (_Cetiosauri_), and of other oviparous quadrupeds of unknown species and genera.