Uf the composition of the cranium.—The occipital bone is developed, as in the placental Mammalia,from four centres or elements,—the basilar below, the supra-occipital above, and the ex-occipitals at the sides; but these ele ments remain longer separate, and in some genera do not become at any period of life united by continuous ossification.
In the skull of an aged Virginian Opossum, I found the supra-occipital still distinct from the ex-occipitals, and these not joined together, though anchylosed to the basilar element. I say not joined together, because in this Mar supial animal they meet above the foramen occipitale and complete its boundaries, as the corresponding superior vertebral laminae com plete the medullary canal in the region of the spine. I find the same structure and condition of the occipital bone of an adult Dasyurus Ursinus, and it is exhibited in the plate of the cranium of this species given by M. Temminck.* In the skull of the mature Wombat, of which a reduced representation is given at fig. 94, the ex-occipitals were still unanchylosed ; the left is figured separate at a.
In the skull of a Perameles nasuta the ex occipitals are separated by an interspace, so that a fissure is continued from the upper part of the foramen magnum to the supra-occipittil element. The same structure may be observe.. in the great Kangaroo, and it is very re markable in the young skulls of this species ; I found this superior notch wide and well marked in Macropus Bennettii. In the Wombat the corresponding fissure is very wide, and the lower margin of the supra-occipital is notched, so that the shape of the foramen magnum somewhat resembles that of the trefoil leaf. In the Koala, the Phalanger, Petaurists, Ilypsi prymni, and Dasyurus Maugei, the elements of the occipital bone present the usual state of bony confluence.
The temporal bone generally presents a per manent separation of the squamous, petrous, and tympanic elements. I have observed this reptilian-like condition of the bone in the ma ture skulls of an Ursine Dasyure, a Virginian Opossum, a Perameles, in different species of Potoroo and Kangaroo, in the Wombat, and in the Koala. The petrous and mastoid elements are commonly anchylosed together. So loose indeed is the connexion of the tympanic bone, that without due care it is very liable to be lost in preparing the skulls of the Marsupials. In the Kangaroo and Wombat (fig. 94, 6) it 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 to a des cending process of the squamous element of the temporal bone. Here we have a near ap
proach to the form of tympanic bone in Birds, but 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 shcwn in the skull of the Porcupine and Beaver, in which the mastoid element sends down a thin obtuse process behind the petro tympanic portion. It is to the expansion of the 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 order, as is well known, a large bulbs own, which is situated anterior and internal to the mastoid process. In many of the Marsupials, as the Dasyures, Petaurists, Pcrameles, Po toroos, and Koala, there is also a large bulbs asses for the purpose of increasing the extent of the auditory cavity ; but, with one single ex ception, the Wombat, this belle is not formed 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. In Acrobates and Perameles lagotis, in addition to the preceding bulla there is also an external dilatation of the petrous element of the tem poral bone, which thus forms a second and smaller bulla on each side, behind the large bulla ossea formed by the sphenoid. In other Marsupials the petrous bone is of smallsize, gene rally 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 usually anchylosed together in the Marsu pials, and the mastoid portion appears in the occipital region of the skull of the Koala, between the ex-occipital hones and squamous portion of the temporal. The petrous element of the temporal bone appears externally in the corresponding part of the skull of a young Emeu. In the Kangaroos and Wombat the petro-mastoid bone presents 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 be tween the basilar bone, ex-occipital and sphe noid, 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 ; but this process is entirely due to the ex-occipitals in the Koala and other Marsupials.