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.—