144 
ELEMENTS OF GEOLOGY. 
recent) is intended to express a minor proportion of recent 
species (of testacea), the term Pliocene (from Ttkuov^ pleion, 
more^ and KaivoQ^ cainos, recent) a comparative plurality of 
the same. It may assist the memory of students to remind 
them, that the Jf^ocene contain a mmor proportion, and Pli¬ 
ocene a comparative phwdXitj of recent species; and that 
the greater number of recent species always implies the more 
modern origin of the strata. 
It has sometimes been objected to this nomenclature that 
certain species of infusoria found in the chalk are still exist¬ 
ing, 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 concholog- 
ical data, and in that sense have always been and are still 
used by me. 
Since the year 1830 the number of known shells, both re¬ 
cent and fossil, has largely increased, and their identification 
has been more accurately determined. Hence some modifi¬ 
cations have been required in the classifications founded on 
less perfect materials. The Eocene, Miocene, and Pliocene 
periods have been made to comprehend certain; sets of strata 
of which the fossils do not always conform strictly in the 
proportion of recent to extinct species with the definitions 
first given by me, or which are implied in the etymology of 
those terms. 
