334 HISTORICAL PALZONTOLOGY. 
CHAPTER, SAL 
THE QUATERNARY PERIOD. 
THE PostT-PLIOCENE PERIOD. 
Later than any of the Tertiary formations are various de- 
tached and more or less superficial accumulations, which are 
generally spoken of as the ost-Zertiary formations, in accord- 
ance with the nomenclature of Sir Charles Lyell—or as the 
Quaternary formations, in accordance with the general usage 
of Continental geologists. In all these formations we meet 
with no Mollusca except such as are now alive—with the 
partial and very limited exception of some of the oldest de- 
posits of this period, in which a few of the shells occasionally 
belong to species not known to be in existence at the pre- 
sent day. Whilst the She//-jish of the Quaternary deposits are, 
generally speaking, identical with existing forms, the JZammads 
are sometimes referable to living, sometimes to extinct species. 
In accordance with this, the Quaternary formations are divided 
into two groups: (1) The fostPiiocene, in which the shells are 
almost invariably referable to existing species, but some of the 
Mammals are extinct ; and (2) the Recent, in which Zhe shells 
and the Mammals alike belong to existing species. The Post- 
Pliocene deposits are often spoken of as the Flecstocene forma- 
tions (Gr. Aleistos, most; ainos, new or recent), in allusion to 
the fact that the great majority of the living beings of this 
period belong to the species characteristic of the ‘‘ new” or 
Recent period. 
The Recent deposits, though of the highest possible interest, 
do not properly concern the palzontologist strictly so-called, 
but the zoologist, since they contain the remains of none but 
existing animals. They are “ Pre-historic,” but they belong 
entirely to the existing terrestrial order. The fost- Pliocene 
deposits, on the other hand, contain the remains of various 
extinct Mammals ; and though Man undoubtedly existed in, 
at any rate, the later portion of this period, if not through- 
out the whole of it, they properly form part of the domain of 
the paleontologist. 
The Post-Pliocene deposits are extremely varied, and very 
widely distributed ; and owing to the mode of their occurrence, 
the ordinary geological tests of age are in their case but very 
partially available. The subject of the classification of these 
