2. _Paucituberculates._--The second sub-order of marsupials, the
Paucituberculata, is exclusively South American, and typically
represented by the family _Epanorthidae_, the majority of the members
of which are extinct, their remains being found in the probably
Miocene Santa Cruz beds of Patagonia, although one existing genus
(_Caenolestes_) survives in Ecuador and Colombia. One of the two
living species was, indeed, described so long ago as the year 1863,
under the preoccupied name of _Hyracodon_, but attracted little or no
attention, as its affinities were not fully recognized. Externally
_Caenolestes_ has a shrew-like appearance. The elongated skull (fig.
8) has four pairs of upper incisors and long upper canines, while in
the lower jaw there is a single pair of procumbent incisors, followed
by several small teeth representing the canine and earlier premolars.
The three pairs of molars in each jaw are, like the last premolar,
quadritubercular oblong teeth. The five-toed feet are of normal
structure, and the rat-like tail is prehensile towards the tip. The
female has a small pouch. The extinct members of the family are
represented by the genera _Epanorthus_, _Acdestis_, _Garzonia_, &c. In
a second family--_Abderitidae_--also from the Patagonian Miocene, the
penultimate premolar is developed into an enormous tooth, with a tall,
secant and grooved crown, somewhat after the fashion of the enlarged
premolar of _Plagiaulax_. From the structure of the skull, it is
thought probable that _Abderites_ had an elongated snout, like that of
many Insectivora. As a sub-order, the Paucituberculata are
characterized by the presence of four pairs of upper and three of
lower incisor teeth; the enlargement and forward inclination of the
first pair of lower incisors, and the presence of four or five sharp
cusps on the cheek-teeth, coupled with the absence of "syndactylism"
in the hind limbs.
[Illustration: From Flower, _Quart. Journ. Geol. Soc._
FIG. 9.--Front view of Skull of the Koala (Phascolarctus cinereus) to
exhibit Diprotodont type of dentition.]
3. _Diprotodonts._--The third and last sub-order of marsupials is the
Diprotodontia, which is exclusively Australasian and includes the
wombats, koala, cuscuses, kangaroos and their relatives. There are
never more than three pairs of upper and one of lower incisors, of
which the middle upper and the single lower pair are large and
chisel-like (fig. 9); the canines are small or absent; the cheek-teeth
have bluntly tuberculate or transversely-ridged crowns in most cases;
and the hind-feet are syndactylous. With one exception, the intestine
has a caecum, and the pouch is large and opens forwards. It should be
added that Professor Elliot Smith has pointed out a certain
peculiarity in its commissures whereby the brain of the diprotodonts
differs markedly from that of the polyprotodonts and approximates to
the placental type. Dr Einar Lönnberg has also recorded certain
adaptive peculiarities in the stomach. Most of the species,
particularly the specialized types, are more or less completely
herbivorous.
The first family, _Phascolomyidae_, is typified by the wombats; but
according to the view adopted by Mr H. Winge, and endorsed by
Professor Max Weber, is also taken to include the koala. In this wider
sense the family may be characterized as follows. The tympanic process
of the alisphenoid bone of the skull is short, not covering the cavity
of the tympanum, nor reaching the paroccipital process. The tail is
rudimentary, the first hind-toe opposable, the first pair of upper
incisors very large, but the second and third either absent or small
and placed partially behind the larger pair; and only five pairs of
cheek-teeth in each jaw. The stomach has a cardiac gland, and the
number of teats is two.
In the wombats (_Phascolomys_) the dentition is i. 1/1, c. 0/0, p. +
m. 5/5, total 24; all the teeth growing from persistent pulps, and the
incisors large and chisel-like, with enamel only on the front surface.
The cheek-teeth strongly curved, forming from the base to the summit
about a quarter of a circle, the concavity being directed outwards in
the upper and inwards in the lower teeth. The first of the series
(which appears to have no predecessor) single-lobed; the other four
composed of two lobes, each subtriangular in section. Limbs equal,
stout and short. Fore-feet with five distinct toes, each furnished
with a long, strong and slightly curved nail, the first and fifth
considerably shorter than the other three. Hind-feet with a very short
nailless first toe, the second, third and fourth toes partially united
by integument, of nearly equal length, the fifth distinct and rather
shorter; all four with long and curved nails. In the skeleton the
second and third toes are distinctly more slender than the fourth,
showing a tendency towards the character so marked in the following
families. Tail rudimentary. Caecum very short and wide, with a
vermiform appendage (see WOMBAT).
In addition to remains referable to the existing genus, the
Pleistocene deposits of Australia have yielded evidence of an extinct
giant wombat constituting the genus _Phascolonus_ (_Sceparnodon_).
The koala, or "native bear" (_Phascolarctus cinereus_), which differs
widely from the wombats in its arboreal habits, is less specialized as
regards its dentition, of which the formula is i. 3/1, c. 1/0, p. + m.
5/5, total 30. Upper incisors crowded together, cylindroidal, the
first much larger than the others, with a bevelled cutting edge (fig.
9). Canine very small; a considerable interval between it and the
first premolar, which is as long from before backwards but not so
broad as the molars, and has a cutting edge, with a smaller parallel
inner ridge. The molar-like teeth slightly diminishing in size from
the first to the fourth, with square crowns, each bearing four
pyramidal cusps. The lower incisors are partially inclined forwards,
compressed and tapering, bevelled at the ends. Cheek-teeth in
continuous series, as in the upper jaw. Fore-feet with the two inner
toes slightly separated from and opposable to the remaining three, all
with strong curved and much compressed claws. Hind-foot (fig. 10) with
the first toe placed far back, large and broad, the second and third
(united) toes considerably smaller than the other two; the fourth the
largest. No external tail. Fur dense and woolly. Ears of moderate
size, thickly clothed with long hair. Caecum very long and dilated,
with numerous folds. Vertebrae: C. 7, D. 11, L. 8, S. 2, Ca. 8. Ribs
eleven pairs (see KOALA).
[Illustration: FIG. 10.--Skeleton of Right Hind-Foot of Koala
(_Phascolarctus cinereus_), showing stout opposable hallux, followed
by two slender toes, which in the living animal are enclosed as far as
the nails in a common integument.]
Here may be noticed three genera of large extinct marsupials from the
Pleistocene of Australia whose affinities appear to ally them to the
wombat-group on the one hand and to the phalangers on the other. The
longest known is _Diprotodon_, an animal of the size of a rhinoceros,
with a dental formula of i. 3/1, c. 0/0, p. 1/1, m. 4/4, total 28. The
first upper incisor very large and chisel-like, molars with prominent
transverse ridges, as in _Macropus_, but without the longitudinal
connecting ridge. Complete skeletons disinterred by Dr E. C. Stirling
indicate that in the structure of the feet this creature presents
resemblances both to the wombats and the phalangers, but is nearer to
the former than to the latter. On the other hand, the considerably
smaller _Nototherium_, characterized by its sharp and broad skull and
smaller incisors, seems to have been much more wombat-like, and may
perhaps have possessed similar burrowing habits.
[Illustration: From Flower, Quart. _Journ. Geol. Soc._