M. Adolphe Brongniart when founding his classification of the fossiliferous strata in reference to their imbedded fossil plants, has placed the cretaceous group in the same division with the tertiary, that is to say, in his "Age of Angiosperms."[xvi-A] This arrangement is based on the fact, that the cretaceous plants display a transition character from the vegetation of the secondary to that of the tertiary periods. Coniferae and Cycadeae still flourished as in the preceding oolitic and triassic epochs; but with these fossils, some well-marked leaves of dicotyledonous trees referred to several species of the genus Credneria, had been found in Germany in the Quadersandstein and Plaener-kalk. Still more recently, Dr. Debey of Aix-la-Chapelle has met with a great variety of other leaves of dicotyledonous plants in the cretaceous flora[xvi-B], of which he enumerates no less than 26 species, some of the leaves being from four to six inches in length, and in a beautiful state of preservation. In the absence of the organs of fructification and of fossil fruits, the number of species may be exaggerated; but we may nevertheless affirm, reasoning from our present data, that in the lower chalk of Aix-la-Chapelle, Dicotyledonous Angiosperms flourished nearly in equal proportions with Gymnosperms; a fact of great significance, as some geologists had wished to connect the rarity of dicotyledonous trees with a peculiarity in the state of the atmosphere in the earlier ages of the planet, imagining that a denser air and noxious gases, especially carbonic acid in excess, were adverse to the prevalence, not only of the quick-breathing classes of animals, (mammalia and birds,) but to a flora like that now existing, while it favoured the predominance of reptile life, and a cryptogamic and gymnospermous flora. The co-existence, therefore, of dicotyledonous angiosperms in abundance with Cycads and Coniferae, and with a rich reptilian fauna comprising the Iguanodon, Ichthyosaurus, Pliosaurus, and Pterodactyl, in the lower cretaceous series tends, like the oolitic mammalia of Stonesfield and Stuttgart, and the triassic birds of Connecticut, to dispel the idea of a meteorological state of things in the secondary periods widely distinct from that now prevailing.