Maxilla inferior.—The lower jaw of the Marsupials is a part of their osseous structure which claims more than ordinary attention in consequence of the discussions to which the ' fossil specimens of this bone discovered in the oolitic strata of Stonesfield have given rise. These specimens, which are well known to the English reader by the figures of them published in the Bridgewater Treatise of Dr. Buckland, and in the Elements of Geology of Mr. Lyell, were regarded by Cuvier as appertaining to the Marsupial series of Mammalia, and to be nearly allied to the genus Didelphis.
This opinion of the great founder of Oryc tological Science has been called in question by other naturalists, and has been more espe cially opposed by Professor De Blainville, who conceives it to be more probable that they be long to a genus of Saurian Reptiles than to the Didelphis or any genus of insectivorous Mammals. I have examined the two speci mens in the possession of Dr. Buckland, the specimen formerly in the collection of Mr. Broderip and now in the British Museum, and that which is preserved in the Museum at York.
The composition of the lower jaw, each ramus of which consists of one piece of bone, the convex condyle, broad and high coronoid pro cess, and the structure and mode of implan tation of the molar teeth, sufficiently attest the mammiferous character of these remains : the size, elevation, and form of the coronoid pro cess of the lower jaw, the process continued from the angle of the ramus, with the tubercular crowns of the molar teeth, indicate the carni vorous and insectivorous character of the spe cies in question. In the presence of canines and the number of the incisors and molars, one of these small Insectivora (Phascolotherium) approaches most nearly to the smaller species of the modern genus Didelphis; while in the structure of the molar teeth, and in the form of the coronoid process, it very closely re sembles the Thylacinus. The number of the molars in the other genus (Thylacolherium) exceeds that of any known ferine Insectivore, placental or marsupial. We have seen, how ever, that the marsupial illyrinecobius possesses nine molars on each side of both upper and lower jaws. Besides the osteological charac ters above alluded to, there is a peculiarity in the lower jaw of the Marsupial animals, which was first indicated by Cuvier in the genus Didelphys, but which is not restricted to that genus. In the carnivorous Marsupials, as the Thylacine, the lower maxillary bone resembles in general form that of the corresponding spe cies in the placental series, as the Dog : a similar transverse condyle is placed low down near the angle of the jaw, on a level with the series of molar teeth ; a broad and strong coronoid process rises high above the condyle, and is slightly curved backwards; there is the same well-marked depression on the exterior of the ascending ramus for the firm implantation of the temporal muscle, and the lower boun dary of this depression is formed by a strong ridge extended downwards and forwards from the outside of the condyle. But in the Dog and other placental Carnivora (the seals ex cepted), a process, representing the angle of the jaw, extends directly backwards from the middle of the above ridge, which process gives precision and force to the articulation of the jaw, and increases the power by which the masseter acts upon the jaw. Now, although the same curved ridge of bone bounds the lower part of the external muscular depression of the ascending ramus in all the Marsupials, it does not in any of them send backwards, or in any other direction, a process correspond ing to that just deScribed in the Dog and other placental Carnivora. The angle of the jaw
itself, in the Marsupials, is as if it were bent inwards in the form of a process encroaching in various shapes, and various degrees of deve lopment in the different Marsupial genera upon the interspace of the rami of the lower jaw. In looking directly upon the lower margin of the jaw, we see, therefore, in place of the mar gin of a vertical plate of bone, a more or less flattened triangular surface extended between the external ridge, and the internal process or inflected angle. This characteristic structure is clearly exemplified in the fossil jaw from the Stonesfield oolitc, in the British Museum, presenting the extinct Marsupial which I have termed Phuscolotherium Bucklandii. In the Opossums the internal angular process is angular and triedral, directed inwards, with the point slightly curved upwards, and more produced in the small than in the large species. In the Dasyures it has a similar form, but the apex is extended into an obtuse process. In the Thylacine the base of the inverted angle is proportionally more extended, and a similar structure is presented by the fossil there. In the Perameles the angle of the jaw forms a still longer process ; it is of a tened form extended obliquely inwards and backwards and slightly curved upwards. It presents a triangular, slightly incurved, and pointed form in the Petuurists, in which it is longest and weakest in the pigmy species, (Acrobates, Desm.) It is shorter and stronger in the Myrmecobius (fig. 97). In the toroos and Phalangers the process is broad with the apex slightly veloped; it is bent inwards and bounds the lower part of a wide and deep depression in the inside of the ascending ramus. In the great Kangaroo the internal margin of this process is curved upwards, so as to augment the depth of the internal depression above tioned. The internal angular cess arrives at its maximum of development in the Wombat, (fig. 94,) and the breadth of the base of the ascending ramus very nearly equals the height of the same part. This broad base also inclines downwards and outwards from the inflected angle, and the samepeculiarity occurs in the jaw of the Phascolothere. In the Koala the size of the process in question is also considerable, but it is compressed, and directed backwards with the obtuse apex only bending inwards, so that the characteristic flattening of the base of the ascending ramus is least marked in this species. There is no depression on the inner side of the ramus of the jaw in the Koala, but its smooth surface is simply pierced near its middle by the dental artery. The surface of the external muscular depression bounded below by a broad angular ridge, as above scribed, is entire in the Dasyures, Opossums, Band/toots, Petaurists, and Phalangers; but in the Wombat the outer surface of the cending ramus is directly perforated by a round aperture immediately posterior to the mencement of the dental canal :* the ponding aperture is of larger size in the garoo. But in the Potoroos both the external and internal depressions of the ascending ramus lead to wide canals, or continuations of the wide depressions which pass forwards into the substance of the horizontal ramus, and soon uniting into one passage, leave a vacant space in the intervening bony septum. This struc ture, if it had been observed only in the jaw of a fossil Marsupial, would have supported an argument for its Saurian nature, more cogent than any that have been adduced in the dis cussion of the Stonesfield fossils, on account of the analogous vacuity in the jaw of the Cro codile.