The first series, the Thallophytes, are simple unicellular plants, and occupy the same position in the vegetable kingdom as the Protozoa do in the animal kingdom. Fossil remains of these organisms seem to be fairly well distributed throughout the entire geological series, but, owing to the soft structure of the fronds in most of the types, it is often a matter of doubt whether we are dealing with a true thallophyte or not. Many of the so-called sea-weeds (fucoids) may be only trails or markings left by other organisms, as shell-fish and crustaceans. BRYOPHYTA.-- The second series, the Bryophytes or moss plants, are represented in the fossil state by a few unimportant examples. PTERIDOPHYTA.-- The third series, the Pteridophytes, includes the Ferns found from the Devonian up to the present day, Horse-tails and allied forms, like _Equisetites_, and the Club-mosses and _Lepidodendron_ of the Carboniferous period in various parts of the world. PTERIDOSPERMEAE.-- The fourth series, the Pteridospermeae, comprises some of the earliest seed-bearing plants, as _Alethopteris_ and _Neuropteris_. They occur in rocks of Upper Palaeozoic age as far as known. GYMNOSPERMEAE. The fifth series, the Gymnospermeae, contains the most important types of plants found fossil, especially those of the primary and secondary rocks: they were more abundant, with the exception of the Coniferae, in the earlier than in the more recent geological periods. ANGIOSPERMEAE.-- The sixth series, the Angiospermeae, comprises all the Flowering Trees and Plants forming the bulk of the flora now living, and is divided into the kinds having single or double seed-leaves (Monocotyledones the Dicotyledones respectively). This important group came into existence towards the close of the Cretaceous period simultaneously with the higher mammals, and increased in abundance until modern times. CHAPTER III. THE GEOLOGICAL EPOCHS: AND THE TIME RANGE OF FOSSILS. =Superposition of Strata.--=