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^vards in the form of a process encroaching in various shapes and va- 

 rious degrees of development, in the difFei ent marsupial genera, upon 

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

 the lower margin of the jaw, we see therefore, in place of the margin 

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

 between the external ridge and the internal process or inflected 

 angle. In the Opossums this internal angular process is triangular 

 and trihedral, directed inwards, with the point slightly curved upwards. 

 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 Phascolothere. In the Perameles the angle of the jaw 

 forms a still longer process ; it is of a flattened form, extended obliquely 

 inwards and backwards, and slightly curved upwards. In the Poto- 

 roos and Phalangers the process is broad, with the apex shghtly de- 

 veloped ; it is bent inwards, and bounds the lower part of a wide 

 and deep depression on the inside of the ascending ramus. In the 

 great Kangaroo the internal margin of this process is curved up- 

 wards, so as to augment the depth of the internal depression above- 

 mentioned. The internal angular process arrives at its maximum 

 of development in the Wombat, and the breadth of the base of the 

 ascending ramus very nearly equals the height of the same ; this 

 broad base also inclines downwards and outwards from the inflected 

 angle, and the same peculiarity occurs in the jaw of the fossil Phas- 

 colothere. In the Koala the size of the process in question is also 

 considerable, but it is compressed, and directed backwards, with the 

 obtuse a])ex only bending inwards, so that the characteristic flatten- 

 ing 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. There is a corresponding perfora- 

 tion on the external surface of the ramus, upon which we observe 

 the external muscular depression bounded below by a broad an- 

 gular ridge. In the Dasyure, there is no external perforation corre- 

 sponding with the dental canal on the inside of the ramus. The 

 ramus is likewise entire in the Petaurists, Phalangers, Perameles, and 

 Opossums. In the Wombat the ascending ramus is directly per- 

 forated by a round aperture immediately posterior to the com- 

 mencement of the dental canal : the corresponding aperture is of 

 larger size in the Kangaroo. But in the Potoroos both the external 

 and internal depressions of the ascending i-amus lead to wide canals, 

 or continuations of the depressions, which pass forwards into the sub- 

 stance of the horizontal ramus, and soon uniting into one passage, 

 leave a vacant space in the intervening bony septum : this structure, 

 if it had existed only in the jaw of a fossil marsupial, would have 

 supported an argument for its Saurian nature, on account of a nearly 

 similar structure in the jaw of the Crocodile. The posterior aperture 

 of the dental canal is situated in the Potoroos and Wombat, as in 

 the Stonesfield fossils, just behind the last raolar tooth ; and in the 

 Wombat a vascular groove is continued from the foramen along the 



