_pleion_, more; _kainos_, new),
with generally _more_ than half the species of shells identical
with existing species--the proportion of these varying from 35
to 50 per cent in the lower beds of this division, up to 90 or
95 per cent in its higher portion.
IV. The _Post-Tertiary Formations_, in which all _the shells
belong to existing species_. This, in turn, is divided into two
minor groups--the _Post-Pliocene_ and _Recent Formations_. In
the _Post-Pliocene_ formations, while all the _Mollusca_ belong
to existing species, most of the _Mammals_ belong to extinct
species. In the Recent period, the quadrupeds, as well as the
shells, belong to living species.
The above, with some modifications, was the original classification
proposed by Sir Charles Lyell for the Tertiary rocks, and now
universally accepted. More recent researches, it is true, have
somewhat altered the proportions of existing species to extinct,
as stated above. The general principle, however, of an increase
in the number of living species, still holds good; and this is as
yet the only satisfactory basis upon which it has been proposed
to arrange the Tertiary deposits.
EOCENE FORMATION.
The Eocene rocks are the lowest of the Tertiary series, and comprise
all those Tertiary deposits in which there is only a small proportion
of existing _Mollusca_--from three and a half to five per cent.
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