ord beds" as truly referable to
this period, we meet at the close of this period with shells such
as nowadays are distinctively characteristic of high latitudes. It
might be thought that the occurrence of Quadrupeds such as the
Elephant, Rhinoceros, and Hippopotamus, would militate against
this generalisation, and would rather support the view that the
climate of Europe and the United States must have been a hot
one during the later portion of the Pliocene period. We have,
however, reason to believe that many of these extinct Mammals
were more abundantly furnished with hair, and more adapted to
withstand a cool temperature, than any of their living congeners.
We have also to recollect that many of these large herbivorous
quadrupeds may have been, and indeed probably were, more or less
migratory in their habits; and that whilst the winters of the
later portion of the Pliocene period were cold, the summers might
have been very hot. This would allow of a northward migration
of such terrestrial animals during the summer-time, when there
would be an ample supply of food and a suitably high temperature,
and a southward recession towards the approach of winter.

The chief palaeontological interests of the Pliocene deposits,
as of the succeeding Post-Pliocene, centre round the Mammals of
the period; and amongst the many forms of these we may restrict
our attention to the orders of the Hoofed Quadrupeds (_Ungulates_),
the _Proboscideans_, the _Carnivora_, and the _Quadrumana_. Almost
all the other Mammalian orders are more or less fully represented
in Pliocene times, but none of them attains any special interest
till we enter upon the Post-Pliocene.

Amongst the Odd-toed Ungulates, in addition to the remains of
true Tapirs (_Tapirus Arvernensis_), we meet with the bones of
several species of Rhinoceros, of which the _Rhinoceros Etruscus_
and _R. Megarhinus_ (fig. 249) are the most important. The former
of these (fig. 249, A) derives its specific name from its abundance
in the Pliocene deposits of the Val d'Arno, near Florence, and
though principally Pliocene in its distribution, it survived
into the earlier portion of the Post-Pliocene period. _Rhinoceros
Etruscus_ agreed with the existing African forms in having two
horns placed one behind the other, the front one being the longest;
but it was comparatively slight and slender in its build, whilst
the nostrils were separated by an incomplete bony partition. In
the _Rhinocero