Cavity of the cranium.—The parietes of the cranial cavity are remarkable for their thickness in some of the Marsupial genera. In the Wombat the two tables of the parietal bones are separated posteriorly for the extent of more than half an inch, the insterspace being filled with a coarse cellular diploe ; the frontal bones are about two and a half lines thick. In the Ursine Dasyure the cranial bones have a si milar texture and relative thickness. In the Koala the texture of the cranial bones is denser, and their thickness varies from two lines to half a line. In the Kangaroo the thickness varies considerably in different parts of the skull, but the parietes are generally so thin as to be dia which is the case with the smaller Marsupials, as the Potoroos and l'etaurists.
The union of the body of the second with that of the third cranial vertebra takes place in the marsupial as in the placental Mammalia at the sella turcica, which is overarched by the back. ward extension of the lesser aim of the sphenoid. The optic foramina and the fissure lacer& anteriores are all blended together, so that a wide opening leads outwards from each side of the sella. Immediately posterior and external to this opening are the foramina rotunda, from each of which in the Kangaroo a remarkable groove leads to the fossa Gasseriana at the com mencement of the foramen ovate ; the same groove is indicated in a slight degree in the Dasyures and Phalangers, but is almost ob solete in the Wombat and Koala. The carotid canals pierce the body of the sphenoid, as in Birds, and terminate in the skull very close together behind the sella turcica, which is not bounded by a posterior clinoid process. The sphenoidal bulla, which forms the chief part of the tympanic cavity in the Perarneles lagotis, forms a large convex protuberance on each side of the door of the cranial cavity in that species. The petrous bone in the Kangaroo, Koala, and Phalangers is impressed above the meatus auditorius by a deep, smooth, round pit, which lodges the lateral appendage of the cerebellum. The corresponding pit is shallower in the I)a syuri, and is almost obsolete in the Wombat. The middle and posterior fissurm lacers: have the usual relative position, but the latter are small. The condyles are each perforated ante riorly by two foramina in most of the Mar. supials, the Thylacinus forming the exception. Of the composition and form of the foramen magnum we have already spoken : it is o' great size in relation to the capacity of the cranium ; the aspect of its plane is backward and slightly downwards.
In the Kangaroo and Phalam,er a thin ridge of bone extends for the distance of one or two lines into the periphery of the tentorial process of the dura mazer, and two sharp spines are sent down into it from the upper part of the cranium in the Phalangista vutpina. The ten torium is supported by a thick ridge of bone in the Thylacine ; but it is not completely ossified in any of the Marsupials : in some species, indeed, as the Dasyures, the Koala, and the Wombat, the bony crista above deH scribed does not exist. There is no ossification) I of the falciform ligament as in the Ornitho rhyncbus.
The anterior depression or olfactory division of the cavity of the cranium, as it may be termed from its large size, is separated in a well-marked manner from the proper cerebral division of the cavity. It is relatively smallest in the Koala. In all Marsupials it is bounded anteriorly by the cribriform plate of the eeth moid bone, which is converted into an osseous reticulation by the number and size of the olfactory apertures. The cavity of the nose, from its great size and the complication of the turbinated bones, forms an important part of the skull. It is divided by a complete bony septum to within one-fourth of the an terior aperture ; the anterior margin of the septum is slightly concave in the Koala; de scribes a slight convex line in the Wombat, 1 Kangaroo, and Phalanger, and a sigmoid 1 flexure in the Dasyure. A longitudinal ridge projects downwards from the inside of each of the nasal bones, and is continued posteriorly . into the superior turbinated bone; this bone extends into the dilated space anterior to the cranial cavity, which corresponds with the frontal sinuses. The convolutions of the middle spongy bone are extended chiefly in the axis of the skull ; the processes of the anterior con voluted bone are arranged obliquely from below upwards and forwards. They are ex tremely delicate and numerous in the Da syures and Phalangers ; they consist of thin laminae of bone beautifully arranged on the convex surface of the os turbinatum, and placed vertically to that surface in the Po toroo ; but the bone becomes very simple in the Kangaroo, Koala, and Wombat. The nasal cavity communicates freely with large maxillary sinuses, and finally terminates by wide apertures behind the bony palate. In the skull the nasal cavity communicates with the .1 mouth, as before mentioned, by means of the various large vacuities in the palatal processes.