They encroach upon the posterior borders of the maxillary plate. Anterior to these vacancies there are two smaller foramina, and posterior to them are one or two similar foramina.
In the Australian Potoroos, Wombat, and Koala, the posterior palatal openings are and oval, and situated entirely in the palatal bones. In Hyps. setosus they extend as far for wards as the interspace between the first and second true molars; in Hyps. nsurinas they reach to that between the second and third true molars. Posterior and external to these large vacuities there are two small perforations. In the Phalangers, with the exception of Ph. Cookii, the palatal openings are proportionally larger; they extend into the palatal process of the maxillaries, and the thin bridge of bone which divides the openings in the Potoroo, &c., is wanting ; the two perforations at the pos terior external angles of the palatine bones are also present. In the Virginian Opossum the bony palate presents eight distinct perforations, besides the incisive foramina ; the palatal pro cesses of the palatine bone extend as far for wards in the median line as the third molars : a long and narrow fissure extends for an equal distance (three lines) into the palatal processes both of the palatines and maxillaries : behind these fissures and nearer the median line are two smaller oblong fissures ; external and a little posterior to these are two similar fissures, situated in the palato-maxillary suture ; lastly, there are two round perforations close to the posterior margin of the bony palate.
In the Ursine Dasyure a large transversely oblong aperture is situated at the posterior part of the palatal processes of the maxillary bones, and encroaches a little upon the palatines ; this aperture is partly,* perhaps in young skulls wholly, bisected by a narrow longitu dinal osseous bridge. In Mange's Dasyure there are two large ovate apertures crossing the palato maxillary sutures separated from each other by a broad plate of bone ; posterior to these are two apertures of similar size and form, which, being situated nearer the mesial line,.are divided by a narrower osseous bridge ; each posterior external angle of the bony palate is also perforated by an oval aperture. In the
Viverrine Dasyure the two vacancies which cross the palato-maxillary suture are in the form of longitudinal fissures, corresponding to the fourth and fifth grinders ; the posterior margin of the bony palate has four small aper tures on the same transverse line.
Now there is no carnivorous quadruped in the placental series which has a bony palate cha racterized by perforations and vacuities of this kind. In the Dog, the Cat, and the Weasel-tribe the bony palate is only perforated by two small oblique canals which open in or near the palato-maxillary suture. The very great in terest which is attached to the fossil remains of the Stonesfield Marsupials, the only mammi ferous remains hitherto discovered in the se condary formations, will justify the minuteness, perhaps tediousness, with which I have dwelt on characters that, inclusive of the teeth, serve to distinguish the cranium of the Marsupial from that of any Placental quadruped. The structure of the bony palate in the Marsupials is interesting in other respects. Since the de fective condition of this part of the cranium is one of the characteristics of the skull of the Bird, it might be expected that some approx imation would be made to that structure in the animals which form the transition between the Placental and Oviparous Classes. We have already noticed the large vacuities which occur in the bony palate of nearly all the Marsupials ; hut this imperfectly ossified condition is most remarkable in the great Peranieles lagotis and Acrobates. In the former ( fig. 96) the bony roof of the mouth is perforated by a wide oval space extending from the second pre molars to the penultimate molars, exposing to view the vomer and the convolutions of the inferior spongy bones in the nasal cavity. Be hind this space there are six small perforations, two in a transverse line midway between the great vacancy and the posterior margin of the bony palate, and four in a transverse line close to that margin. In Acrobates a still larger pro portion of the posterior part of the palate is formed by membrane.