As the first example of the group may be taken the elegant little
long-snouted phalanger (_Tarsipes rostratus_, fig. 12), a west
Australian creature of the size of a mouse, which may be regarded as
representing by itself a sub-family (_Tarsipediinae_), characterized
by the rudimentary teeth, the long and extensile tongue, and absence
of a caecum. The head is elongated, with a slender muzzle and the
mouth-opening small. The two lower incisors are long, very slender,
sharp-pointed and horizontally placed. All the other teeth are simple,
conical, minute and placed at considerable and irregular intervals
apart in the jaws, the number appearing to vary in different
individuals and even on different sides of the jaw of the same
individuals. The formula in one specimen was i.(2 - 2)/(1 - 1), c.(1 -
1)/(0 - 0), p. + m.(3 - 4)/(2 - 3); total 20. The lower jaw is
slender, nearly straight, and without a coronoid process or inflected
angle. Fore-feet with five well-developed toes, carrying small, flat,
scale-like nails, not reaching the extremity of the digits. Hind-feet
rather long and slender, with a well-developed opposable and nailless
first toe; second and third digits united, with sharp, compressed
curved claws; the fourth and fifth free, with small flat nails. Ears
of moderate size and rounded. Tail longer than the body and head,
scantily clothed with short hairs, prehensile. Vertebrae: C. 7, D. 13,
L. 5, S. 3. Ca. 24.
As indicated in the accompanying illustration, the long-snouted
phalanger is arboreal in habits, extracting honey and probably small
insects from long-tubed flowers by means of its extensile tongue.
The remaining members of the family may be included in the sub family
_Phalangerinae_, characterized by the normal nature of the dentition
(which shows rudimentary lower canines) and tongue. Cuscuses and
phalangers form a numerous group, all the members of which are
arboreal, and some of which are provided with lateral expansions of
skin enabling them to glide from tree to tree like flying-squirrels.
The typical members of the group are the cuscuses (_Phalanger_),
ranging from the Moluccas and Celebes to New Guinea, in which the
males are often different in colour from the females. The true
phalangers, or opossums of the colonists, constitute the genus
_Trichosurus_, while the ring-tailed species are known as
_Pseudochirus_; the latter ranging to New Guinea. _Dactylopsila_ is
easily recognized by its attenuated fourth finger and parti-coloured
fur; the flying species are classed as _Petauroides_, _Petaurus_,
_Gymnobelideus_ and _Acrobates_, the last no larger than a mouse;
while Dromicia, _Distaechurus_ and _Acrobates_ are allied types
without parachutes (see PHALANGER).
An equally brief notice must suffice of the kangaroo tribe or
_Macropodidae_, since these receive a special notice elsewhere. The
dentition is i.(3/1) c.(0 or 1)/0 p.(3/3) m.(3/3); the incisors being
sharp and cutting, and those of the lower jaw frequently having a
scissor-like action against one another. The broad molars are either
bluntly tuberculated or transversely ridged; the outer side of the
hind part of the lower jaw has a deep pocket; and the hind-limbs are
generally very long, with the structure of the foot similar to that of
the bandicoots. The family is connected with the _Phalangeridae_ by
means of the musk-kangaroo (_Hypsiprymnodon moschatus_); forming the
sub-family _Hypsiprymnodontinae_. Then come the rat-kangaroos, or
kangaroo-rats, constituting the sub-family _Potoroinae_; while the
tree-kangaroos (_Dendrolagus_), rock-wallabies (_Petrogale_), and
wallabies and kangaroos (_Macropus_) form the _Macropodinae_ (see
KANGAROO).
_Extinct Marsupials_
Reference has been made to the Australasian Pleistocene genera
_Phascolonus_, _Diprotodon_, _Nototherium_ and _Thylacoleo_, whose
affinities are with the wombats and phalangers. The same deposits have
also yielded remains of extinct types of kangaroo, some of gigantic
size, constituting the genera _Sthenurus_, _Procoptodon_ and
_Palorchestes_. Numerous types more or less nearly allied to the
phalangers, such as _Burramys_ and _Triclis_ have also been described,
as well as a flying form, _Polaeopetaurus_. It is also interesting to
note that fossil remains indicate the former occurrence of thylacines
and Tasmanian devils on the Australian mainland. Of more interest is
the imperfectly known _Wynyardia_, from older Tertiary beds in
Tasmania, which apparently presents points of affinity both to
phalangers and dasyures. From the Oligocene deposits of France and
southern England have been obtained numerous remains of opossums
referable to the American family _Didelphyidae_. These ancient
opossums have been separated generically from _Didelphys_ (in its
widest sense) on account of certain differences in the relative sizes
of the lower premolars, but as nearly the whole of the species have
been formed on lower jaws, of which some hundreds have been found, it
is impossible to judge how far these differences are correlated with
other dental or osteological characters. In the opinion of Dr H.
Filhol, the fossils themselves represent two genera, _Peratherium_,
containing the greater part of the species, about twenty in number,
and _Amphiperatherium_, with three species only. All are comparatively
small animals, few of them exceeding the size of a rat.
Besides these interesting European fossils, a certain number of
didelphian bones have been found in the caves of Brazil, but these are
either closely allied to or identical with the species now living in
the same region.
The occurrence in the Santa Cruz beds of Patagonia of fossil
marsupials allied to the living _Caenolestes_ has been mentioned
above. The alleged occurrence in the same beds of marsupials allied to
the thylacine is based on remains now more generally regarded as
referable to the creodont carnivores (see CREODONTA).
_Mesozoic Mammals._--Under the heading of MULTITUBERCULATA will be
found a brief account of certain extinct mammals from the Mesozoic
formations of Europe and North America which have been regarded as
more or less nearly related to the monotremes. The same deposits have
yielded remains of small mammals whose dentition approximates more
nearly to that of either polyprotodont marsupials or insectivores; and
these may be conveniently noticed here without prejudice to their true
affinities. Before proceeding further it may be mentioned that the
remains of many of these mammals are very scarce, even in formations
apparently in every way suitable to the preservation of such fossils,
and it hence seems probable that these creatures are stragglers from
a country where primitive small mammals were abundant. Not improbably
this country was either "Gondwana-land," connecting Mesozoic India
with Africa, or perhaps Africa itself. At any rate, there seems little
doubt that it was the region where creodonts and other primitive
mammals were first differentiated from their reptilian ancestors.
[Illustration: From Owen.