The auditory chamber of the ear is aug mented in the Phalangers, the Koala, the Kan garoos, and Potoroo, by a continuation of air cells into the base or origin of the zygomatic process ; but the extent of the bony air-cham bers communicating with the tympanum is proportionally greatest in the Flying Opossums, where, besides the sphenoid bulla, the mastoid element and the whole of the zygomatic pro cess 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 is directed outwards and a little forwards in the carnivorous Dasyures ; out wards and a little backwards in the Pcrameles and Plialangers ; outwards, backwards, and upright in the Kangaroos, and directly out wards in the Petaurists and Wombat; but the differences of direction are but 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 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 received 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 jaw. 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 relation 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 oQ 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 a concavity still more elongated transversely, less deep than in the placental Carnivora, but adapted, as in them, to a ginglymoid motion of the lower jaw. In all the genera, save in the Wombat, retraction of the lower jaw is opposed by a descending process of the temporal bone immediately anterior to the meatus auditorius and tympanic bone.
The glenoid cavity presents a characteristic structure in most of the Marsupialia in not I being exclusively formed by the temporal bone. With the exception of the Petaurists, the malar bone forms the outer part of the articular sur face for the lower jaw, and in the Thylacinus, Dasyurus Maugei, Dasyurus ursinus, Pera 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.