_ generally reaches half-way from the root of the 
zygoma to the sagittal ridge or suture; it is 
most developed in the Wombat, in which its 
_ superior margin describes a remarkably straight 
line. The zygomatic process of the temporal 
bone is generally compressed and much ex- 
_ tended in the vertical direction in the Opossum, 
_ Dasyure, Phalanger, Koala, and Kangaroo. 
| In the Wombat it curves outwards from the 
_ side of the head in the form of a compressed 
and almost horizontal plate; it is then sud- 
_ denly twisted into the vertical position, to be 
_ Feceived into the notch of the malar portion of 
the arch. 
| _ The cavity corresponding to the sphenoidal 
bulla ossea in other Marsupials is in this 
| Species excavated in the lower part of the 
_ Squamous element of the temporal bone at the 
_ inner side of the articular surface for the lower 
‘ er This articular surface, situated at the 
__ base of the zygomatic process, presents in the 
| Marsupial, as in the placental Mammalia, 
_ various forms, each manifesting a physiological 
_ Telation to the structure of the teeth and adapted 
| to the required movement of the jaws in the 
_ Various genera. In the herbivorous Kangaroo 
_ the glenoid cavity forms a broad and slightly 
_ convex surface, as in the Ruminants, affording 
freedom of rotation to the lower jaw in every 
direction. In the Phalangers and Potoroos 
the articular surface is quite plane. In the 
_ Perameles it is slightly convex from side to 
_ side, and concave from behind forwards. In 
_ the Wombat it is formed by a narrow convex 
_ ridge considerably extended, and slightly con- 
_ Cave, in the transverse direction. This ridge is 
| not bounded by any descending process pos- 
| teriorly, so that the jaw is left free for the 
“movements of protraction and retraction. But 
‘this structure is widely different from that 
which facilitates similar movements in the Ro- 
dentia. In these there is a longitudinal groove 
on each side, in which the condyle of the lower 
_ jaw plays backwards and forwards, but is im- 
_ peded in its lateral movements; these, on the 
contrary, are freely allowed to the Wombat, 
| and the oblique disposition of, the lines of 
enamel upon the molar teeth correspond with 
the various movements of which the lower jaw 
of the Wombat is thus susceptible. In the 
Koala the glenoid cavity is a transversely ob- 
_ long depression with a slight convex rising at 
the bottom, indicating rotatory movements of 
the jaw. In the carnivorous Dasyures it forms 
4 concavity still more elongated transversely, 
tess deep than in the placental Carnivora, but 
adapted, as in them, toa ginglymoid motion of 
_ the lower jaw. In all the genera, save in the 
i, Wombat, retraction of the lower jaw is opposed 
_ bya descending process of the temporal bone 
_ immediately anterior to the meatus auditorius 
and tympanic bone. 
_ The glenoid cavity presents a characteristic 
tructure in most of the Marsupialia in not 
ing exclusively formed by the temporal bone. 
ith the exception of the Petaurists, the malar 
one forms the outer part of the articular sur- 
ce for the lower jaw, and in the Thylacinus, 
lasyurus Maugei, Dasyurus ursinus, Pera- 
MARSUPIALIA. 
f 271 
meles, Hypsiprymnus, and Macropus the sphe- 
noid ala forms the inner boundary of the same 
surface; but this.ala does not extend so far out- 
wards and backwards in the Wombat or Koala. 
The sphenoid bone has the same general 
form and relative position as in the ordinary 
Mammalia, but in many Marsupials it presents 
a similarity to that in the Ovipara in the per- 
sistence of the pterygoid processes as separate 
bones, as shown in the Wombat (fig. 94, c). 
It is only in the Koala that I have observed 
a complete obliteration of the suture joining 
the basilar element of the sphenoid with 
that of the occipital bone. In the Thylacine 
a narrow straight bridge of bone is continued 
from the auditory sphenoidal bulla forwards to 
the base of the pterygoid process, resembling 
the condition of the pterygoids in Birds. 
The chief peculiarity in the sphenoid bone 
is the dilatation of the root of the great ala 
already alluded to. This dilatation communi- 
cates and is filled with air from the tympanum. 
It forms the hemispherical bulla ossea on each 
side of the basis cranii in the Dasyures and 
Phascogales, and the large semi-ovate bull in 
the Myrmecobius, Cook’s Phalanger, &c.; but 
in the Koala the bulle (6, fig. 95,) are still 
( iN AIT x 
Phascolarcius. 
more developed, and are produced downwards 
to an extent equal with the ex-occipital pro- 
cesses (a, fig. 95); they are somewhat com- 
pressed laterally, and instead of the smooth 
and polished surface which characterizes them 
in the preceding genera, terminate here in a 
rough ridge. The dilated air-chambers or 
bull of the sphenoid are very small in the 
Thylacine; in the Phalangers and Potoroos 
they are relatively smaller than in the Dasyures, 
and they are incomplete posteriorly in the Kan- 
garoos and Wombat. In the Brush-Kangaroo 
the above process from the sphenoid joins the 
base of the large descending process of the 
ex-occipital. The pterygoid processes are 
relatively largest in the Kangaroo, Wombat, 
and Koala, and present in each of these species 
distinct hamular processes. In the Potoroo, 
Kangaroo, and Wombat the sphenoid ala com- 
bines with the pterygoid process to form a 
large and deep depression opening externally. 
In the Kangaroo, Dasyures, Koala, and Wom- 
bat the great ale of the sphenoid articulate 
with the parietal bones, but by a very small 
portion in the two latter species: in the Pera- 
