e other hand, numerous
remains of Quadrupeds have been brought to light, representing
most of the great Mammalian orders now in existence upon the
earth, and in many cases indicating animals of very considerable
dimensions. We are, in fact, in a position to assert that the
majority of the great groups of Quadrupeds with which we are
familiar at the present day were already in existence in the
Eocene period, and that their ancient root-stocks were even in
this early time separated by most of the fundamental differences
of structure which distinguish their living representatives.
At the same time, there are some amongst the Eocene quadrupeds
which have a "generalised" character, and which may be regarded
as structural types standing midway between groups now sharply
separated from one another.

The order of the _Marsupials_--including the existing Kangaroos,
Wombats, Opossums, Phalangers, &c.--is poorly represented in
deposits of Eocene age. The most celebrated example of this group
is the _Didelphys gypsorum_ of the Gypseous beds of Montmartre,
near Paris, an Opossum very nearly allied to the living Opossums
of North and South America.

No member of the _Edenates_ (Sloths, Ant-eaters, and Armadillos)
has hitherto been detected in any Eocene deposit. The aquatic order
of the _Sirenians_ (Dugongs and Manatees), with their fish-like
bodies and tails, paddle-shaped forelimbs, and wholly deficient
hind-limbs, are represented in strata of this age by remains of
the ancient "Sea-Cows," to which the name of _Halitherium_ has
been applied. Nearly allied to the preceding is the likewise aquatic
order of the Whales and Dolphins (_Cetaceans_), in which the body
is also fish-like, the hind-limbs are wanting, the fore-limbs are
converted into powerful "flippers" or swimming-paddles, and the
terminal extremity of the body is furnished with a horizontal,
tail-fin. Many existing Cetaceans (such as the Whalebone Whales)
have no true teeth; but others (Dolphins, Porpoises, Sperm Whales)
possess simple conical teeth. In strata of Eocene age, however, we
find a singular group of Whales, constituting the genus _Zeuglodon
(fig. 228), in which the teeth differed from those of all existing
forms in being of two kinds,--the front ones being conical incisors,
whilst the back teeth or molars have serrated triangular crowns,
and are inserted in the jaw by two roots. Each molar (fig. 228,
A) looks as if it were composed of two separate teeth unit