Mr. Owen on the Thylacotherium. 51 



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 Marsupialia, it does not in any of 

 them send backwards, or in any other direction, a process corresponding to that 

 just described in the Dog and other placental Carnivora. The angle of the jaw 

 itself, in the Marsupials, is bent inwards in the form of a process, encroaching 

 in various shapes and degrees of development, in the different marsupial genera, 

 upon the interspace of the rami of the lower jaw. In looking directly upon 

 the lower edge of the jaw, we see, therefore, in place of the margin of a vertical 

 plate of bone, a more or less flattened triangular surface or plate of bone extended 

 between the external ridge and the internal process or inflected angle. In the 

 Opossums this internal angular process is triangular and trihedral, directed in- 

 wards, 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 Perameles it forms a still longer 

 process of a flattened form, extended obliquely inwards and backwards, and 

 slightly curved upwards. It presents a triangular, slightly incurved and pointed 

 form in the Petaurists, in which it is longest and weakest in the Pigmy species 

 (Acrobates, Desm.). In the Potoroos and Phalangers the process is broad, with 

 the apex slightly developed. 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-mentioned. The internal angular pro- 

 cess arrives at its maximum of development in the Wombat, so that the breadth of 

 the base of the ascending ramus very nearly equals the height of the same part. 



Now if the process from the angle of the jaw in the Stonesfield fossil be ac- 

 tually continued backwards, as M. Valenciennes describes, in the axis, or nearly so, 

 of the ramus of the jaw, then such fossil would resemble the jaw of an ordinary 

 placental carnivorous or insectivorous mammal, and the term Thylacotherium would 

 be inappropriately applied to it ; but in both the specimens of the fossil jaws to 

 which that name has been restricted, and which present their inner or mesial sur- 

 faces to the observer, the angular process (PI. V. figs. 1 and 3, c) presents a frac- 

 tured surface ; and we are led to conclude, therefore, that when the jaw was en- 

 tire its angle was produced inwards or mesially ; but whether to the same extent 

 as in the Opossum and other Marsupials, or with a slight inclination, as in the 

 Mole and Hedgehog, cannot be certainly determined in the present fossils. In these 

 specimens, however, the apex of the angle only is fractured ; while in the fossil 



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