The Ilypsiprymni come next in the order of development of the zygomatic arches ; which again are proportionally much stronger in the true Kangaroos. The length of the zygomata in relation to the entire skull is greatest in the Koala and Wombat. In the former animal they are remarkable for their depth and straight and parallel course, as well as for their longi tudinal extent. In the Wombat they have a considerable curve outwards, so as greatly to diminish the resemblance which otherwise ex ists in the form of the skull between this Mar supial and the herbivorous Rodentia of the placental series, as, e. g. the Viscaccia.
In the carnivorous Marsupials the outward sweep of the zygomatic arch, which is greatest in the Thylacine and Ursine Dasyure, is also accompanied by a 'slight curve upwards, but this curvature is chiefly expressed by the con cavity of the lower margin of the zygoma, and is by no means so well marked as in the placental Carnivore. It is remarkable that this upward curvature is greater in the slender zygomata of the l'erameles than in the stronger zygomata of the Dasyures and Opossums. In the Koala and Phalangers there is also a slight tendency to the upward curvature; in the Wombat the outwardly expanded arch is per , fectly horizontal. In the Kangaroo the lower margin of the zygoma describes a slightly undu lating curve, the middle part of which is con vex downwards.
In many of the Marsupials, as the Kangaroo, the Koala, some of the Phalangers, Petaurists,; and Opossums, the superior margin of the zygoma begins immediately to rise above the posterior origin of the arch. In the Wombat an external ridge of bone commences at the middle of the lower margin of the zygoma, and gradually extends outwards as it advances for wards, and being joined by the upper margin of the zygoma, forms the lower boundary of the orbit, and ultimately curves downwards in front of the ant-orbital foramen, below which it bifurcates and is lost. This ridge results, as it were, from the flattening of the anterior part of the zygoma, which thus forms a smooth and slightly concave horizontal platform for the eye to rest upon.
The same structure obtains, but in a slighter degree, in the Koala.
In the Kangaroo the anterior and inferior part of the zygoma is extended downwards in the form of a conical process, which reaches below the level of the grinding-teeth. A much
shorter and more obtuse process is observable in the corresponding situation in the Phalangers and Opossums.
The relative length of the facial part of the skull anterior to the zygomatic arches varies re. markably in the different Marsupial genera. In the Wombat it is as six to nineteen ; in the Koala as five to fourteen ; in the Pe taurus sciureus and Petaurus Bennettii it forms about one-fourth of the entire skull ; in the Phalangers about one-third ; in the carnivorous Dasyures and Opossums more than one-third ; in the Thylacine nearly one-half; in Peraineles, Macropus, and Hypsiprymnus murinus, Ill. the length of the skull anterior to the orbit is equal to the remaining posterior part ; but in a species of Hypsiprymnus from Van Dieman's Land (Hypsiprymnus myosurus, Ogilb.), the facial part of the skull anterior to the orbit exceeds that of the remainder, and the arboreal Hypsi prymni from New Guinea present a still greater length of muzzle. In most Marsupials the skull gradually converges towards the anterior extremity ; the convergence is more sudden in the Petaurists, especially Pet. Bennettii; but in the Perameles lagotis the skull is re markable for the sudden narrowing of the face anterior to the orbits, and the prolongation of the attenuated snout, preserving the same diameter for upwards of an inch before it finally tapers to the extremity of the nose. In the Koala the corresponding part of the skull is as remarkable for its shortness, as it is in the Per. lagotis for its length, but it is bounded laterally by parallel lines through its whole extent. Before concluding this account of the general form of the skull, I may observe that in nearly all the Marsupials two long processes project downwards from the inferior angles of the occipital region ; they correspond in func tion with, and have been desrnbed as the mastoids, but they are developed from the ex-occipital bones. These processes are longest in the Kangaroos and Koala; in the Wombat they co-exist with the true mastoid pro cesses, which are of larger size. In the Opos sums and Dasyures the exoccipital processes are short and obtuse ; in Acrobates they cease to exist, but they are present in the larger Petaurists.