shells of this period could be referred to living species, so that
this era seemed to indicate the dawn of the present testaceous
fauna, no living species of shells having been detected in the
antecedent or Secondary rocks.

Some conchologists are now unwilling to allow that any Eocene
species of shell has really survived to our times so unaltered as
to allow of its specific identification with a living species. I
cannot enter in this place into this wide controversy. It is enough
at present to remark that the character of the Eocene fauna, as
contrasted with that of the antecedent Secondary formations, wears
a very modern aspect, and that some able living conchologists still
maintain that there are Eocene shells not specifically
distinguishable from those now extant; though they may be fewer in
number than was supposed in 1833.

The term Miocene (from Greek meion, less; and Greek kainos, recent)
is intended to express a minor proportion of Recent species (of
testacea); the term Pliocene (from Greek pleion, more; and Greek
kainos, recent), a comparative plurality of the same.

It has sometimes been objected to this nomenclature that certain
species of infusoria found in the chalk are still existing, and, on
the other hand, the Miocene and Older Pliocene deposits often
contain the remains of mammalia, reptiles, and fish, exclusively of
extinct species. But the reader must bear in mind that the terms
Eocene, Miocene, and Pliocene were originally invented with
reference purely to conchological data, and in that sense have
always been and are still used by me.