384 PROF. OWEN ON THE OSTEOLOGY 
by the tympanic, or any other element of the temporal bone, but by the expansion of 
the base of the great ala of the sphenoid bone. It is only in the Perameles lagotis’ 
that, in addition to the preceding bulla, I have observed an external dilatation of the 
petrous element of the temporal bone, which thus forms a second and smaller bulla on 
each side, behind the large bulla ossea formed by the sphenoid. In other Marsupiata 
the petrous bone is of small size, generally limited to the office of protecting the parts 
of the internal ear, and sometimes, as in the Koala, is barely visible at the exterior of 
the base of the skull. The petrous and mastoid elements are commonly anchylosed 
together. In the Kangaroos, Koala and Wombat, the petro-mastoid bone is of a larger 
size, and is visible in two situations on the outside of the skull, viz. at the usual place 
at the basis, where the petrous portion is wedged in between the basilar bone, ex- 
occipital and sphenoid ; and again at the side of the cranium, where the mastoid portion 
appears between the squamous, ex-occipital and supra-occipital bones. 
In the Wombat it sends outwards the strong compressed process which terminates the 
lateral boundaries of the occipital plane of the cranium. The auditory chamber of the 
ear is augmented in the Phalangers, the Koala, the Kangaroo and Potoroo, by a con- 
tinuation of air-cells into the base or origin of the zygomatic process ; but the extent of 
the bony air-chambers communicating with the tympanum is proportionally greatest in 
the Petaurists, or Flying Opossums, where, besides the sphenoid bulla, the mastoid ele- 
ment, and the whole of the zygomatic process of the temporal bone are expanded to form 
air-cells with very thin and smooth walls ; thus presenting an interesting analogy in the 
structure of the cranium to the class of Birds. The direction of the bony canal of the 
organ of hearing corresponds, as in the placental Mammalia, with the habits of the 
species: the meatus auditorius is directed outwards and a little forwards in the car- 
nivorous Dasyures ; outwards, and a little backwards in the Perameles and Phalanger ; 
outwards, backwards and upwards in the Kangaroos ; and directly outwards in the 
Petaurists and Wombat ; but the differences of direction are slightly marked. 
The squamous element of the temporal bone 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 in general compressed, and much extended in the vertical direc- 
tion 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 depressed and almost 
horizontal plate ; it is then suddenly twisted into the vertical position, to be received 
in the notch of the malar portion of the arch. The cavity, corresponding to the sphe- 
noidal 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 jaw. This articular surface, situated at the base of the zygomatic process, 
| Pl, LXXI. fig. 1. 
