OSTEOLOGY OF TOE MARSUPIALIA.
Of the Skull.—The form of the skull varies much in different Marsupial animals, but it may be said, in general terms, to re semble an elongated cone, being terminated by a vertical plane surface behind, and in most of the species converging towards a point ante riorly : it is also generally more depressed or flattened than in the placental Mammalia. The skull is also remarkable in all the Marsupial genera for the small proportion which is de voted to the protection of the brain, and for the great expansion of the nasal cavity immediately anterior to the cranial cavity.
In the stronger carnivorous Marsupials the exterior of the cranium is characterized by bony ridges and muscular impressions, but in the smaller herbivorous and insectivorous species, as the Petaurists, Potoroos, and Myrraccobius, the cranium presents a smooth convex surface as in Birds, corresponding with the smooth un convoluted surface of the simple brain con tained The breadth of the skull in relation to its length is greatest in the Wombat,• Ursine Da syuret and Petaurists, in which it equals three fourths of the length, and is least in the Pere metes lagolis, in which it is less than one-half.
The occipital region, which is generally plane, and vertical in position, forms a right angle with the upper surface of the skull, from which it is separated by an occipital or lambdoidal .crista. This crista is least developed in the Myrmecobius, Petaurists, and Kangaroos, and most so in the Thylacine and larger Opossums, in which, as also in the Koala, the crest curves slightly back wards, and thus changes the occipi tal plane into a concavity for the firm implanta tion of the strong muscles from the neck and back.
The upper surface of the skull presents great diversity of character, which relates to the dif ferent development of the temporal muscles, and the varieties of dentition in the different genera.
Itt the Wombat the corona! surface offers an almost flattened tract bounded by two slightly elevated temporal ridges, which are upwards of an inch apart posteriorly, and slightly diverge, as they extend forwards to the anterior part of the orbit.
The skull of the Virginian Opossum pre sents the greatest contrast to that condition, for the sides of the cranium meet above at an acute angle, and send upwards from the line of their union a remarkably elevated sagittal crest, wh ich, in mature skulls, is proportionally more deve lopet than in any of the placental Carnivore, not even excepting the strong-jawed Ifywna.
The Thylacine and Dasyures, especially the Ursine Dasyure, exhibit the sagittal crest in a somewhat less degree of development. It is again smaller, but yet well marked in the Koala and Perameles. The temporal ridges meet at the lambdoidal suture in the larger and in the Hypsiprynani, but the size of the muscles in these does not require the deve lopment of a bony crest.
In the Kangaroo, the temporal ridges, which are very slightly raised, are separated by an interspace of the third of an inch.
They are separated for a proportionally greater extent in the Petaurists, especially Petaunts flavivenler ; and in the smooth and convex upper surface of the skull of Petaurus seiuretts, Pet. pignueus, illynnecobius, the im pressions of the feeble temporal muscles almost cease to be discernible.
The zygomatic arches are, however, complete in these as in all the other genera : they are usually, indeed, strongly developed ; but their variations do not indicate the nature of the food so clearly, or correspond with the diffe rences of animal and vegetable diet in the same degree as in the placental Marnmalia. And this is not surprising when we recollect that no Marsupial animal is devoid of incisors in the upper jaw, like the ordinary Ruminants of the placental series : accordingly the more com plete dental system with which the herbivorous Kangaroos, Potoroos, Phalangers, &c. are pro vided, and which appears to be in relation to the scantier pasturage and the dry and rigid character of the herbage or foliage on which they browse, requires a stronger apparatus of bone and muscle for the action of the jaws, and especially for the working of the terminal teeth. There are, however, well marked diffe rences in this part of the Marsupial skull ; and the weakest zygomatic arches are those of the Insectivorous Perameles and Acrobates, in which structure we may discern a correspon dence with the Edentate Anteaters of the pla cental series. Still the difference in the deve lopment of the zygomata is greatly in favour of the Marsupial Insectivore.