160 THE SKULL. [chap. 



in the Capybara, Viscacha, Aguti, Paca, &c.) it is narrow 

 and concave transversely, with prominent inner and outer 

 edges, the latter being often formed, as before mentioned, 

 partly by the hinder end of the malar. In the Beaver the 

 glenoid fossa has considerable breadth. In the Porcupines, 

 Marmots, Squirrels, Rats, &c, no raised inner margin is 

 developed, and the fossa passes insensibly into the side of 

 the skull wall. In the Hare it is a transversely oval hollow, 

 with a prominent rounded anterior margin. 



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 tym- 

 panic cavity, apparently in the periotic. In some genera 

 (Pedetes, Dipus, Chinchilla) this attains an enormous size 

 (see Fig. 55, 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. 



