principally distinguished from one another by the different size and
shape of the permanent premolar tooth, or that which displaces and
succeeds the deciduous molars in the vertical direction. This tooth
is displayed in its closed alveolus in both specimens(l) in which
situation, notwithstanding their superiority of size over the largest
existing Kangaroos, I was led to seek for it, by observing the sharp,
unworn summits of the crowns of the molar teeth and other signs of
immaturity in the fossil specimens.
The total number of molar teeth successively developed in the
great extinct Kangaroos is the same as in the existing species, viz :
two deciduous molars, one premolar, and four true molars : and the
permanent series of five appears to have been longer retained than in
the large existing species.
The true molars of the upper jaw in the Macropus Titan differ
from those of the Macr. Atlas and of all the existing species in having
a well-developed ridge at the back part of the base of the crown in
addition to the two principal transverse- eminences and the anterior
basal ridge. In the Macropus Atlas the posterior basal talon of the
upper molars is much smaller, the crown broader, especially its
anterior division, and the ridge connecting the two transverse eminences
is shorter and more simple. In the lower jaw the molars
likewise present modifications characteristic of the two species : those
of the Macropus Titan have no posterior basal ridge, but the anterior
one is longer, as is also the ridge connecting the two chief transverse
eminences ; and the antero-posterior extent of the crown is greater in
proportion to its breadth, than in the Macropus Atlas: these characters
are shown in the figure of the left penultimate molar, PI. 101,
fig. 2 j, the greater thickness of the transverse ridges is shown in
fig. 1.
In the lower molars of Macropus Atlas the posterior talon exists
in the same rudimental state as in the upper jaw, and the anterior
talon is shorter than in the Macr. Titan, as is shown in the right
penultimate molar in PI. 101, fig. 4. The upper premolar of the 1
(1) They are now preserved in the Museum of the Geological Society, and were originally
described by me in Sir T. Mitchell’s “ Expeditions into Australia.’’ 8vo.PI. 29. 1838>, Vol ii- vd 361 >
Macr. Atlas has the same remarkable size as the lower one displayed
in fig- 3, PL 101 ; the anterior end of the crown is irregularly
notched, the inner surface uniform, and its margin entire ; the outer
surface is obliquely indented, forming a notched lobe posteriorly.
The outer surface of the lower premolar is more equally divided by
an oblique vertical fissure into two lobes. The relative size of this
tooth to the true molars is not only greater than in the Macropus
Titan, but also than in the existing true Kangaroos, {Macropus),
and clearly indicates a subgeneric type connecting these with the
Potoroos, {Hypsiprymnus). The large procumbent flattened inferior
incisor displays the characteristic form of that tooth in the existing
Kangaroos, • and has the same strengthening ridge along its inner
size, fig. 5.
154. Rhizophaga.—In this tribe, the stomach is simple in outward
form, but complicated within by a large cardiac gland ; and the
cæcum, which is short and wide, is furnished with a vermiform
appendage.
Genus Phascolomys, (PI. 100, fig. 9.)—In its heavy shapeless
figure, large trunk, and short equably developed legs, the Wombat
offers as great a contrast to the Kangaroos as does the Koala, which
it most nearly resembles in its general outward form and w’ant of
tail. But in the more important characters afforded by the teeth
and intestinal canal, the Wombat differs more from the Koala than
the latter does from either the Phalangers or Kangaroos.
The dental system presents the extreme degree of that degradation
of the teeth, intermediate between the front incisors and true
molars, which we have been tracing from the Opossum to the
Kangaroos : not only have the functionless premolars and canines
now totally disappeared, but also the posterior incisors of the upper
jaw, which we have seen in the Potoroos to exhibit a feeble degree of
development as compared with the anterior pair ; these in fact are
alone retained in the dentition of the Wombat, which is thus reduced
to that of the true Rodentia.
Incisors y ; canines ; premolars ^ ; molars : = 24.
The incisors, moreover, are true dentes scalprarii with persistent