270 
Phascolomys. 
forms a complete bony tube, about half an 
inch in length, with an irregular exterior ; 
it is wedged in between the mastoid and 
articular processes of the temporal bone. In 
the Potoroo the bony circle is incomplete at 
the upper part; in the Perameles and Da- 
syures the tympanic bone forms a semicircle, 
the posterior part being deficient, and the tym- 
panic membrane being there attached toa des- 
cending process of the squamous element of 
the temporal bone. Here we have a near ap- 
ingen to the form of tympanic bone in Birds, 
ut we have a still closer resemblance to its 
condition both in Birds and Reptiles, in its 
want of union with and relations to the petrous 
element of the temporal bone. In the Rodent 
quadrupeds the tympanic, petrous, and mas- 
toid elements of the temporal bone are always 
anchylosed together; this condition is well 
shewn in the skull of the Porcupine and 
Beaver, in which the mastoid element sends 
down a thin obtuse preares behind the petro- 
Ppa portion. It is to the expansion of 
e petro-tympanic, and not of the mastoid 
portion of the temporal bone, that the enlarge- 
ment of the tympanic cavity is due in the 
Rodentia, and this expansion forms in that 
MARSUPIALIA. 
‘the auditory cavity ; but, with one single 
order, as is well known, a bulla osse 
which is situated anterior and internal tot 
mastoid process. In many of the Marsupi: 
as the Dasyures, Petaurists, Perameles, F 
toroos, and Koala, there is also a large bu 
ossea for the purpose of increasing the exter 
ception, the Wombat, this bulla is not fon 
by the tympanic or any other element of 
temporal bone, but by the expansion 
base of the great ala of the sphenoid be 
Acrobates and Perameles lagotis, in addi 
to the preceding bulla there is also an ext 
dilatation of the petrous element of the 
poral bone, which thus forms a second 
smaller bulla on each side, behind t 
bulla ossea formed by the sphenoid. r 
Marsupials the petrous bone is of smalls: 
rally limited to the office of protecting the pp 
of the internal ear, and sometimes, as 
Koala, is barely visible at the exterior of 
of the skull. The petrous and mastoid elem 
are usually anchylosed together in the Mi 
pials, and the mastoid portion appears 
occipital region of the skull of the Ke 
between the ex-occipital bones and squan 
portion of the temporal. The petrous elen 
of the temporal bone ap external 
the corresponding part of the skull of a yo 
Emeu. In the Kangaroos and Womk 
petro-mastoid bone presents a larger size, 
is visible in two situations on the outsid 
the skull, viz. at the usual place at the 
where the petrous portion is wedged in 
tween the basilar bone, ex-occipital and s 
noid, and again at the side of the cra 
where the mastoid portion appears between 
squamous, ex-occipital, and supra-oee 
bones. In the Wombat it sends outwards 
strong compressed process which 
the lateral boundaries of the occipital pla 
the cranium ; but this process is entirely 
to the ex-occipitals in the Koala and « 
Marsupials. 
The auditory chamber of the ear is | 
mented in the Phalangers, the Koala, the’ 
garoos, and Potoroo, by a continuation ¢ 
cells into the base or origin of the 
haga de but the extent of the bony air-el 
ers communicating with the tympanu 
proportionally greatest in the Flying Oposs 
where, besides the phen bulla, the m 
element and the whole of the zygom 
cess of the temporal bone are expandé 
form air-cells with very thin and smooth ¥ 
thus presenting an interesting anal r 
structure of the cranium to the class of bir 
The direction of the bony canal of 
of hearing corresponds, as in the pla 
Mammalia, with the habits of the sp 
The meatus is directed outwards and a 
forwards in the carnivorous Dasyures, 
wards and a little backwards in the Peram 
and Phalangers; outwards, backwaré 
upright in the Kangaroos, and directly 
wards in the Petaurists and Wombat; but 
differences of direction are but slightly mai 
The squamous element of the tempe 
