SKELETON OF MAESUPIALIA. 349 



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 INIarsupials, it does not in any of them send 

 backward, or in any other direction, a process corresponding to 

 that just described in the Dog. The angle of the jaw itself, 

 in the Marsupials, is as if it were bent inward in the form of 

 a process, encroaching in various shapes and various degrees of 

 developement in the different genera upon the interspace of the 

 rami of the lower jaw. On looking directly 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 triangular sur- 

 face extended between the external ridge, and the internal pro- 

 cess or inflected angle. In the Opossums the internal angular 

 process is triangular and trihedral, directed inward, with the 

 point slightly curved upward, 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 Thy- 

 lacine the base of the inverted angle is proportionally more 

 extended, and a similar structure is presented by the fossil Phas- 

 colothere. In the Perameles the angle of the jaw forms a still 

 longer process ; it is of a flattened form extended 

 obliquely inward and backward and slightly curved ^"^ 

 upward. It presents a triangular, slightly incurved, 

 and pointed form in the Petaurists, in which it is longest 

 and weakest in the pigmy species {^Acrohates, Desm.). 

 It is shorter and stronger in the Myrmecobius, fig. 

 223, a. In the Potoroos and Phalangers the process 

 is broad with the apex slightly developed ; it is bent 

 inward and bounds the loAver part of a wide and 

 deep depression in the inside of the ascending ramus. 

 In the Great Kano-aroo the internal marcrin of this 

 process is turned upward, so as to augment the depth 

 of the internal depression above mentioned. The in- 

 ternal angular process arrives at its maximum of de- 

 velopement in the Wombat, fig. 220, «, and the breadth L.nvorjaw, 

 of the base of the ascending ramus very nearly equals 

 the height of the same part. In the Koala the size of the process 

 in question is also considerable, but it is compressed, and directed 

 backward with the obtuse apex only bending inward, 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 



