266 LOWER CRETACEOUS AND WEALDEN FLORA. [Ch. XVIIL 



triassic epochs ; but, together with these, some well-marked leaves of 

 dicotyledonous trees, of a genus named Credneria, have long been known. 

 They are met with in the " quader-sandstein" and " planer-kalk" of Ger- 

 many, rocks of the Upper Cretaceous group. More recently, Dr. Deby 

 has discovered in the Lower Cretaceous beds of Aix-la-Chapelle a great 

 variety of dicotyledonous leaves,* belonging to no less, according to his 

 enumeration, 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 certainly affirm, reasoning from our 

 present data, that when the lower chalk of Aix-la-Chapelle originated, 

 Dicotyledonous Angiospenns flourished in that region in equal proportions 

 with Gymnosperms. This discovery has an important bearing on some 

 popular theories, for until lately none of these Exogens (a class now con- 

 stituting three-fourths of the living plants of the globe) had been detected 

 in any strata older than the Eocene. Moreover, some geologists have 

 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 gas being 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 ex- 

 isting, while it favored the predominance of reptile life, and a cryptogamic 

 and gymnospermous flora. The coexistence, therefore, of Dicotyledonous 

 Angiospenns in abundance with Cycads and Coniferae, and with a rich 

 reptilian fauna, comprising the Iguanodon, Megalosaurus, Hylseosaurus, 

 Ichthyosaurus, Plesiosaurus, and Pterodactyl, in the Lower Cretaceous se- 

 ries, tends manifestly to dispel the idea of a meteorological state of things 

 in the secondary periods so widely distinct from that now prevailing. 



Among the recent additions made to the fossil flora of the Wealden, 

 and one which supplies a new link between it and the tertiary flora, I 

 may mention the Gyrogonites, or spore-vessels of the Chara, lately found 

 in the Hastings series of the Isle of Wisht. 



much cited, it may be useful to geologists to give a table explaining the corre- 

 sponding names of groups so much spoken of in palaeontology. 



Brongniart. 



.2 r 



a 



as 

 f 



b 



c 



1. Cryptogamous am- J 



phigens, or cellular > 

 cryptogamic. ) 



2. Cryptogamous acro- 



gens. 



3. Dicotyledonous gym- 



nosperms. 

 4 Dicot. Angiospenns. 



Lindley. 



Thallogens. 

 Acrogens. 



Gymnogens. 

 Exogens. 



Lichens, sea- weeds, fuDgi. 



Mosses, equisetums, ferns, lyco- 

 podiuins, — Lepidodendron. 



Conifers and Cycads. 



Compositae, leguminosse, umbel- 

 liferse, cruciferae, heaths, <fcc. 

 All native European trees ex- 

 cept conifers. 

 5. Monocotyledons. Endogens. Palms, lilies, aloes, ruhes, grasses, 



<fcc. 

 * Geol. Quart. Jour. vol. vii. part 2, Miscell. p. 111. 



