t the older are the progenitors of the more recent
forms, while, in some cases, they distinctly favour that hypothesis.
The period in time and the changes in physical geography represented
by the nummulitic deposits are undoubtedly very great, while the
remains of Middle Eocene and Older Eocene Mammals are comparatively
few. The general facies of the Middle Eocene Fauna, however, is quite
that of the Upper. The Older Eocene pre-nummulitic mammalian Fauna
contains Bats, two genera of _Carnivora_, three genera of _Ungulata_
(probably all perissodactyle), and a didelphid Marsupial; all these
forms, except perhaps the Bat and the Opossum, belong to genera
which are not known to occur out of the Lower Eocene formation. The
_Coryphodon_ appears to have been allied to the Miocene and later
Tapirs, while _Pliolophus_, in its skull and dentition, curiously
partakes of both artiodactyle and perissodactyle characters; the third
trochanter upon its femur, and its three-toed hind foot, however,
appear definitely to fix its position in the latter division.

There is nothing, then, in what is known of the older Eocene mammals
of the Arctogaeal province to forbid the supposition that they stood
in an ancestral relation to those of the Calcaire Grossier and the
Gypsum of the Paris basin, and that our present fauna, therefore, is
directly derived from that which already existed in Arctogaea at the
commencement of the Tertiary period. But if we now cross the frontier
between the Cainozoic and the Mesozoic faunae, as they are preserved
within the Arctogaeal area, we meet with an astounding change, and
what appears to be a complete and unmistakable break in the line of
biological continuity.

Among the twelve or fourteen species of _Mammalia_ which are said to
have been found in the Purbecks, not one is a member of the orders
_Cheiroptera_, _Rodentia_, _Ungulata_, or _Carnivora_, which are so
well represented in the Tertiaries. No _Insectivora_ are certainly
known, nor any opossum-like Marsupials. Thus there is a vast negative
difference between the Cainozoic and the Mesozoic mammalian faunae
of Europe. But there is a still more important positive difference,
inasmuch as all these Mammalia appear to be Marsupials belonging to
Australian groups, and thus appertaining to a different distributional
province from the Eocene and Miocene marsupials, which are
Austro-Columbian. So far as the imperfect materials which exist enable
a judgment to b