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 have existed, and Monocotyledonous plants or
Endogens are very poorly represented. With the Upper Cretaceous,
however, a new era of plant-life, of which our present is but
the culmination, commenced, with a great and apparently sudden
development of new forms. In place of the Ferns, Cycads, and
Conifers of the earlier Mesozoic deposits, we have now an
astonishingly large number of true Angiospermous Exogens, many
of them belonging to existing types; and along with these are
various Monocotyledonous plants, including the first examples of
the great and important group of the Palms. It is thus a matter
of interest to reflect that plants closely related to those now
inhabiting the earth, were in existence at a time when the ocean
was tenanted by Ammonites and Belemnites, and when land and sea
and air were peopled by the extraordinary extinct Reptiles of
the Mesozoic period.
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