x.] RODENTIA. 183 



The tympanic is ankylosed to the periotic, but not to the 

 squamosal ; it generally develops a tubular meatus, which in 

 the Hare is directed upwards and backwards, in the Beaver, 

 outwards and forwards. In the Porcupines, as well as in 

 most of the smaller Rodents, the meatus is short. In the 

 Capybara it is fissured below. 



There is always a considerable tympanic bulla, which is 

 often supplemented by a bulla developed above the tympanic 

 cavity apparently in the periotic. In some genera (Pedetes, 

 Difus, Chinchilla'] this attains an enormous size (see Fig. 

 59, Per), and forms a rounded prominence on the posterior 

 external angle of the skull, interposed between the squamosal, 

 parietal, and occipital. Usually, the mastoid portion of the 

 periotic only appears on the surface for a small space in front 

 of the exoccipital. In the Beaver, it forms a conspicuous 

 angular process. 



The periotic is never ankylosed with any of the bones of 

 the cranium, other than the tympanic. On its inner surface 

 the floccular fossa is nearly always wide and deep, but it is 

 absent, or nearly so, in the Capybara, Paca, and Porcupine. 

 The place of attachment of the hyoid arch is an inconspicuous 

 depression in the usual situation, and the tympanohyal is 

 never distinct. This is in relation with the rudimentary 

 condition of the anterior cornu of the hyoid. 



In the mandible, the symphysial portion is narrow, curving 

 upwards, and rounded laterally, with a single large alveolus 

 on each side. The coronoid process is never large, and is 

 often rudimentary or absent, in relation to the small develop- 

 ment of the temporal fossa and muscle ; while the portion 

 adjacent to the angle is greatly developed, showing marked 

 masseteric and pterygoid fossae, and often with its lower 

 edge expanded laterally, or slightly incurved. The angle is 

 rounded in the Hares, but it is more often produced into a 



