as _Cycadopteris_), Screw Pines
(_Pandanus_), Oaks (_Quercus_), Walnut (_Juglans_), Fig (_Ficus_),
and many _Proteaceoe_, some of which are referred to existing
genera (_Dryandra, Banksia, Grevillea_, &c.)

[Footnote 23: The "Flowering plants" are divided into the two
great groups of the Endogens and Exogens. The _Endogens_ (such
as Grasses, Palms, Lilies, &c.) have no true bark, nor rings of
growth, and the stem is said to be "endogenous;" the young plant
also possesses but a single seed-leaf or "cotyledon." Hence these
plants are often simply called "_Monocotyledons_." The _Exogens_,
on the other hand, have a true bark; and the stem increases by
annual additions to the outside, so that rings of growth are
produced. The young plant has two seed-leaves or "cotyledons,"
and these plants are therefore called "_Dicotyledons_." Amongst the
Exogens, the Pines (_Conifers_) and the Cycads have seeds which
are unprotected by a seed-vessel, and they are therefore called
"_Gymnosperms_." All the other Exogens, including the ordinary
trees, shrubs, and flowering plants, have the seeds enclosed in
a seed-vessel, and are therefore called "_Angiosperms_." The
derivation of these terms will be found in the Glossary at the
end of the volume.]

In North America, the Cretaceous strata of New Jersey, Alabama,
Nebraska, Kansas, &c., have yielded the remains of numerous plants,
many of which belong to existing genera. Amongst these may be
mentioned Tulip-trees (_Liriodendron_), Sassafras (fig. 186),
Oaks (_Quercus_), Beeches (_Fagus_), Plane-trees (_Platanus_),
Alders (_Alnus_), Dog-wood (_Cornus_), Willows (_Salix_), Poplars
(_Populus_), Cypresses (_Cupressus_), Bald Cypresses (_Taxodium_),
Magnolias, &c. Besides these, however, there occur other forms
which have now entirely disappeared from North America--as, for
example, species of _Cinnamomum_ and _Araucaria_.

It follows from the above, that the Lower and Upper Cretaceous
rocks are, from a botanical point of view, sharply separated
from one another. The Palaeozoic period, as we have seen, is
characterised by the prevalance of "Flowerless" plants
(_Cryptogams_), its higher vegetation consisting almost exclusively
of Conifers. The Mesozoic period, as a whole, is characterised
by the prevalence of the Cryptogamic group of the Ferns, and
the Gymnospermic groups of the Conifers and the Cycads. Up to
the close of the Lower Cretaceous, no Angiospermous Exogens are
certainly known to h