The chief peculiarity in the sphenoid bone is the dilatation of the root of the great ala already alluded to. This dilatation communi cates and is filled with air from the tympanum. It forms the hemispherical bulla ossea on each side of the basis cranii in the Dasyures and Phascogales, and the large semi-ovate bulln in the Myrinecobins, Cook's Phalanger, &c.; but in the Koala the bulhe (b, fig. 95,) are still more developed, and are produced downwards to an extent equal with the ex-occipital pro cesses (a, fig: 95); they are somewhat com pressed laterally, and instead of the smooth and polished surface which characterizes them in the preceding genera, terminate here in a rough ridge. The dilated air-chambers or bulls' of the sphenoid are very small in the Thylacine; in the Phalangers and Potoroos they are relatively smaller than in the Dasyures, and they are incomplete posteriorly in the Kan garoos and Wombat. In the Brush-Kangaroo the above process from the sphenoid joins the base of the large descending process of the ex-occipital. The pterygoid processes are relatively largest in the Kangaroo, Wombat, and Koala, and present in each of these species distinct hamular processes. In the Potoroo, Kangaroo, and Wombat the sphenoid ala com bines with the pterygoid proces.s to form a large and deep depression opening externally. In the Kangaroo, DI asyures, Koala, and Wom bat the great aim of the sphenoid articulate with the parietal bones, but by a very small portion in the two latter species: in the Pera metes and Potoroos the sphenoid alga do not reach the parietals.
There is little to notice in the parietal bones except the obliteration of the sagittal suture in those species in which a bony crista is deve loped in the corresponding place. They pre sent a singularly flattened form in the Wombat, in an aged skull of which, and in a similar one in the Kangaroo, I observe a like obliteration of the sagitta! suture. In the Kantraroo, Potoroo, Petatirus, Phalaneer, and Myrme cobius there is a uiangular inter-parietal bone. The corresponding bone I find in three pieces in the skull of a Wombat.
The frontal buries are chiefly remarkable for their anterior expansion and the great share which they take in the formation of the nasal cavity. In the Thylacine the part of the cranium occupied by the frontal sinuses exceeds in breadth the cerebral cavity, from which it is divided by a constriction. The corona] suture presents in most of the Marsupials an irregular angular course, forming a notch in the frontals on each side which receives a corresponding triangular process of each, parietal bone : this form of the suture is least pronounced in the Acrebates and .11/yrtnecobius. A process cor responding to the posterior frontal augments the bony boundary of the orbit in the Thylacine, the Ursine Dasyure, and in a slighter degree in the Virginian Opossum ; it is relatively most developed in the skull of the Myrinecobies where the orbit is large; but the bony boundary of the orbit is not complete in any of the Marsupials. In the Myrmecobius
there is a deep notch at the middle of the supra-orbital ridge. A corresponding but shallower notch is present in the skull of Petaurus sciureus. I have found the frontal suture obliterated in old specimens of the Thylacine, the Virginian Opossum, Cook's Pha langer, the taguanoid, and yellow-bellied Pe taurists; but the frontal suture exists in Petaurns Sciureus, ilcrobates, and other Marsupials. The inter-orbital space is concave in the Pha hangers and in the Petatirus Taguanoides, but is quite flat in the other I'etaurists.
The lachrymal bones vary in their relative size in different Marsupialia. In the Koala they extend upon the face about a line beyond the anterior boundary of the orbit, and at this part they present a groove with one large and two or three small perforations. In the Wom bat their extent upon the face is slightly in creased ; it is proportionally greater in the Kangaroos, Potoroos, Phalangers, Petaurists, and Dasyures, in which this part of the lachrymal bone presents two perforations close to the orbit. In the Thylacine, besides the two external holes there is a large perforation within the orbital margin. This carnivorous Marsupial, as compared with the Wolf, pre sents a greater extent of the facial portion of the lachrymal bone, and thus indicates its inferior type. In the illyrmecobius the lachry mal bone exhibits its greatest relative develop ment.
The malar bones are very strong and of great extent in almost all the Marsupialia. They are least developed in Aerobeles, Myrinc cobius, and Pcrameles lagotis. In the latter the malar bone presents a singular form, being bifurcate at both extremities the processes zygomuticus mural& superioris is wedged into the cleft of the anterior fork ; the correspond ing process of the temporal bone fills up the posterior notch ; the lower division of this bifurcation is the longest, and in all the Mar-. supialia enters into the composition of the articular surface for the lower jaw, except in the Petaurists, where it just falls short of this part. The anterior bifurcation of the malar bone is not present in the Marsupials generally; the external malo-maxillary suture forms an oblique and almost straight line in the Wombat, Phalanger, Opossum, Dasyures, and Kangaroo. Owing to the inferior development of the zygo matic process of the superior maxillary in the Wombat, the malar bone is not suspended in the zygomatic arch in this Marsupial as in the placental ltodentia. It is also of relatively much larger size and of a prismatic form, arising from the development of the oblique external ridge above described. In the Kan garoos, Potoroos, Great Petaurus, and Plia hangers it is traversed externally by a ridge showing the extent of attachment of the masseter ; in the Koala the ridge extends along the malar bone near the upper margin, and the surface below presents a well-marked excava tion.