se the
hypothesis that the older are the progenitors of the more recent forms,
while, in some cases, they distinctly favour that hypothesis. The period
in tine 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 _Carivora_, 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