Wombat the outwardly expanded arch is per- 
fectly horizontal. In the Kangaroo the lower 
margin of the zygoma describes a slightly undu- 
lating curve, the middle part of which is con- 
vex downwards. 
_ In many of the Marsupials, as the Kangaroo, 
_ the Koala, some of the p 
and Opossums, the superior margin of the 
_ zygoma begins immediately to rise above the 
_ posterior origin of the arch. In the Wombat 
an external ridge of bone commences at the 
_ middle of the lower margin of the zygoma, and 
gradually extends outwards as it advances for- 
wards, and being joined by the upper margin 
of the zygoma, forms the lower boundary of 
‘the orbit, and ultimately curves downwards in 
front of the ant-orbital foramen, below which 
it bifurcates and is lost. This ridge results, as 
it were, from the flattening of the anterior part 
of the zygoma, which thus forms a smooth and 
slightly concave horizontal platform for the eye 
_ to rest upon. 
__ The same structure obtains, but in a slighter 
degree, in the Koala. 
_ In the Kangaroo the anterior and infenor 
part of the zygoma is extended downwards in 
‘the form of a conical process, which reaches 
below the level of the grinding-teeth. A much 
shorter and more obtuse process is observable 
in the corresponding situation in the Phalangers 
and Opossums. 
The relative length of the facial part of the 
skull anterior to the zygomatic arches varies re. 
‘markably in the different Marsupial genera. In 
the Wombat it is as six to nineteen; in 
the Koala as five to fourteen; in the Pe- 
_ taurus sciureus and Petaurus Bennettii it forms 
‘about one-fourth of the entire skull; in the 
_ Phalangers about one-third ; in the carnivorous 
Dasyures and Opossums more than one-third ; 
in the Thylacine nearly one-half; in Perameles, 
Macropus, and Hypsiprymnus murinus, I]. the 
length of the skull anterior to the orbit is equal 
to the remaining posterior part ; but in a species 
Hypsiprymnus from Van Dieman’s Land 
_( Aypsiprymnus myosurus, Ogilb.), the facial 
part of the skull anterior to the orbit exceeds 
that of the remainder, and the arboreal Hypsi- 
ag from New Guinea present a still greater 
length of muzzle. In most Marsupials the 
skull gradually converges towards the anterior 
extremity; the convergence is more sudden 
in the Petaurists, especially Pet. Bennettii ; 
but in the Perameles lagotis the skull is re- 
markable for the sudden narrowing of the face 
anterior to the orbits, and the prolongation of 
the attennated snout, preserving the same 
diameter for upwards of an inch before it finally 
tapers to the extremity of the nose. In the 
Koala the corresponding part of the skull is 
as remarkable for its shortness, as it is in the 
Per. lagotis for its length, but it is bounded 
laterally by parallel lines through its whole 
extent. Before concluding this account of the 
general form of the skull, I may observe that 
in nearly all the Marsupials two long processes 
project downwards from the inferior angles of 
the occipital region ; they correspond in func- 
Vy 
MARSUPIALIA. 
halangers, Petaurists,’ 
269 
tion with, and have been desfribed as the 
mastoids, but they are developed from the 
ex-occipital bones. These processes are longest 
in the Kangaroos and Koala; in the Wombat 
they co-exist with the true mastoid pro- 
cesses, which are of larger size. In the Opos- 
sums and Dasyures the exoccipital processes 
are short and obtuse; in Acrobates they cease 
to exist, but they are present in the larger 
Petaurists. 
Of the composition of the cranium.—The 
occipital bone is developed, as in the placental 
Mammalia, from four centres or elements,—the 
basilar below, the supra-occipital above, and 
the ex-occipitals at the sides; but these ele- 
ments remain longer separate, and in some 
genera do not become at any period of life 
united by continuous ossification. 
In the skull of an aged Virginian Opossum, 
I found the supra-occipital still distinct from 
the ex-occipitals, and these not joined together, 
though anchylosed to the basilar element. I 
say not joined together, because in this Mar- 
supial animal they meet above the foramen 
occipitale and complete its boundaries, as the 
corresponding superior vertebral lamine com- 
plete the medullary canal in the region of the 
spine. I find the same structure and condition 
of the occipital bone of an adult Dasyurus 
Ursinus, and it is exhibited in the plate of 
the cranium of this species given by M. 
Temminck.* 
In the skull of the mature Wombat, of which 
a reduced representation is given at fig. 94, the 
ex-occipitals were still unanchylosed ; the left 
is figured separate at a. 
In the skull of a Perameles nasuta the ex- 
occipitals are separated by an interspace, so 
that a fissure is continued from the upper part 
of the foramen magnum to the supra-occipital 
element. The same structure may be observe: 
in the great Kangaroo, and it is very re- 
markable in the young skulls of this species ; I 
found this superior notch wide and well marked 
in Macropus Bennettiit. In the Wombat the 
corresponding fissure is very wide, and the 
lower margin of the supra-occipital is notched, 
so that the shape of the foramen magnum 
somewhat resembles that of the trefoil leaf. In 
the Koala, the Phalanger, Petaurists, Hypsi- 
prymni, and Dasyurus Maugei, the elements of 
the occipital bone present the usual state of bony 
confluence. 
The temporal bone generally presents a per- 
manent separation of the squamous, petrous, 
and tympanic elements. I have observed this 
reptilian-like condition of the bone in the ma- 
ture skulls of an Ursine Dasyure, a Virginian 
Opossum, a Perameles, in different species of 
Potoroo and Kangaroo, in the Wombat, and in 
the Koala. The petrous and mastoid elements 
are commonly anchylosed together. So loose 
indeed is the connexion of the tympanic bone, 
that without due care it is very liable to be lost 
in preparing the skulls of the Marsupials. In 
the Kangaroo and Wombat (fig. 94, 6) it 
* Monographies de Mammalogie, pl. viii. 
