340 



ANATOMY OF VERTEBRATES. 



220 



ossea for increasing the extent of the auditory cavity, formed by 

 the expansion of the base of the sphenoid. In Acrohates and Pera- 



meles lagotis, there is also an 

 external dilatation of the 

 petrosal, fig. 222, 16, which 

 thus forms a second and 

 smaller bulla on each side, 

 behind the large bulla ossea 

 formed by the alisphenoid, h. 

 In other Marsupials the pe- 

 trous bone is of small size, 

 generally limited to the 

 office of protecting the parts 

 of the internal ear, and 

 sometimes, as in the Koala, 

 is barely visible at the ex- 

 terior of the base of the 

 skull. The mastoid portion 

 appears in the occipital re- 

 gion of the skull of the 



Koala, fig. 221, 8, between 

 the exoccipital and squa- 

 mous j)ortion of the tempo- 

 ral. In the Wombat the 

 mastoid sends outward the 

 strong compressed process, 

 fig. 220, 8, which terminates 

 the lateral boundaries of the 

 occipital plane of the cra- 

 nium ; but this process is 

 entirely due to the exocci- 

 pitals in the Koala, fig. 221, 

 4, and other Marsupials. 

 The auditory chamber of the ear is augmented in the Phalan- 

 gers, the Koala, the Kangaroos, and Potoroo, by a continuation 

 of air-cells into the base or origin of the zygomatic process ; but 

 the extent of the bony air-chambers communicating with the 

 tympanum is proportionally greatest in the Flying Opossums, 

 where, besides the sphenoid bulla, the mastoid and the Avhole of 

 the zygomatic process of the squamosal are expanded to form air- 

 cells with very thin and smooth Avails, fig. 219. 



The direction of the bony canal of the organ of hearing corre- 

 sponds with the habits of the species. The meatus is directed 



Phascolomys. 



