11. The number of fossil species has been estimated at between 3000 and 4000; but many parts of plants are described as separate species, and even genera, and hence the number is perhaps greater than it ought to be.
12. Brongniart divides the fossil flora into three great epochs:—1. The reign of Acrogens; 2. The reign of Gymnosperms; 3. The reign of Angiosperms.
13. The reign of Acrogens embraces the Silurian, Carboniferous, and Permian epochs, in which there was a predominance of plants belonging to the natural orders Filices, Lycopodiaceæ, and Equisetaceæ, associated, however, with others of a higher class.
14. The reign of Gymnosperms embraces the lower and middle Secondary periods, and is characterised by the presence of numerous Coniferæ and Cycadaceæ.
15. The reign of Angiosperms includes the Cretaceous and Tertiary periods, and is marked by the predominance of Angiospermous Dicotyledons.
16. Coal is a vague term, referring to all kinds of fuel formed from the chemically-altered remains of plants.
17. When there is a great admixture of mineral matter, so that it will not burn as fuel, then a shale is produced.
18. The microscopic structure of Coal probably varies according to the nature of the plants of which it is composed, and the changes produced by pressure, heat, and other causes. Cellular tissue, punctated woody tissue, and scalariform vessels, have been detected in it.
19. Certain temporary and local floras seem to have given origin to peculiar layers of coal.
20. During the Carboniferous epoch we meet with Ferns, Sigillarias, and their roots called Stigmarias, Lepidodendrons, Ulodendrons, Calamites, Gymnosperms, etc.