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

 riousdegrees of development, in the differentmarsupial genera, upon 

 the interspace of the rami of the lower ja\v. In looking do\vn upon 

 the lo\ver raargin of the jaw, \ve see therefore, in place of the margin 

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

 between the extemal ridge and the internal process or injlected 

 angle. In the Opossums this internal angular process is triangular 

 and trihedral.directed in\vards, with the point slightlj' 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 backvvards, and slightly curved upwards. In the Poto- 

 roos and Phalangers the process is broad, with the apex slightly 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 vexy nearly eąuals the height of the šame ; this 

 broad base also inclines downw ards and outwards from the inflected 

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

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

 considerable, but it is compressed, and directed backwards, -vvith the 

 obtuse apex only bending inwards, so that the characteristic flatten- 

 ingof the base ofthe ascending rffWi?/s 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- 

 tioH on the external surface of the ramus, upon "which we observe 

 the external muscular depression bounded beloTv by a broad an- 

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

 sponding -yvith 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 ramus lead to ^vide canals, 

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

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

 leave a vacant space in the inten'ening 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 stinicture in the jaw of the Crocodile. The posterior aperture 

 of the dental canal is situated in the Potoroos and \Vombat, as in 

 the Stonesfield fossils, just behind the lašt molar tooth ; and in the 

 Worabat a \-ascular groove is continued from the foramen along the 



