Beginning with the Devonian period of the Paleozoic era the records show that important advances had taken place in the evolution of the plant kingdom. Among the very simple Thallophyte plants some seaweeds of unusually large size occur in fossil form, but the important fact is that all the principal subdivisions of the typical higher non-flowering plants (Pteridophytes) as well as Pteridosperms, and even some primitive representatives of the lower order flowering plants (gymnosperms) were well represented in the Devonian. Our knowledge of land plants earlier than the Devonian amounts to almost nothing and they certainly could not have been at all prominent, but the fossil records make it very clear that many Devonian land areas were clad with rich and diversified plant life. There were even forests, probably the first on earth, but they were far different, both in general and in particular, from those of to-day because the trees were all of exceptionally low organization types. During the next two periods--Mississippian and Pennsylvanian--there was no really important progress in the evolution of plants, and since these remarkable types of land plants have left such wonderfully preserved records in strata of the Pennsylvanian or great Coal Age, we shall proceed to descriptions of the main types of that time, especially those which contributed to the formation of beds of coal. As shown by the abundant records, the land plant life of Pennsylvanian time must have been not only prolific but exceedingly varied. Thousands of species have been unearthed from the coal-bearing formations alone, and these must represent only a fraction of all species of plants which lived during the period. Most prominent of all were the giant Lycopods constituting the lowest main subdivision of the Pteridophytes (see above classification). These great, non-flowering plants were at once the biggest, most common and conspicuous trees of the extensive swamp forests, and they were the greatest contributors to the formation of coal (Plate 15). Many species have been described. They commonly attained heights of 50 to 100 feet and diameters of 2 to 6 feet. In one important type the fairly numerous branches bristled with stiff, needle-shaped leaves. When the leaves dropped off the older or trunk portions, scars were left spirally arranged around the trunks of the trees. In another important type the leaf scars were vertically arranged on the lower portions of the tree trunks. The upper portions of the trunks (rarely branched) were thickly set with long, slender leaves, which in some species were two or three feet long. An interesting fact is that the inner parts of the trunks of the great Lycopods were filled with soft, pithy material. This explains why the fossil trees are nearly always flattened out, as a result of burial within the earth. The nonbranching type of Lycopod has been totally extinct for millions of years, while the branching type is to-day