392 THE VERTEBRATE SKELETON. 



on the supra-occipital, and the tentorium (fig. 72, 21), a 

 transverse fold of the dura mater 1 , separates the large cerebral 

 fossa from the cerebellar fossa, which is much smaller than 

 the cerebral fossa and lies behind and partly beneath it. The 

 plane of the tentorium is called the tentorial plane, and the 

 angles that it makes with the basicranial axis and with the 

 occipital and ethmoidal planes vary much in different mammals. 



The periotic has its inner surface marked by important 

 depressions, while both inner and outer surfaces are pierced 

 by foramina. At about the middle of its inner surface are 

 seen two deep pits, one lying immediately above the other. 

 Of these the more ventral is a foramen, the internal audi- 

 tory meatus (fig. 72, 20), through which the Vllth (facial) 

 and Vlllth (auditory) nerves leave the cranial cavity, the 

 facial nerve passing through the bone and afterwards leaving 

 the skull by the stylomastoid foramen (fig. 75, VII), while 

 the auditory passes to the inner ear. The more dorsal of the 

 two pits is not a foramen but the floccular fossa (fig. 72, 23) 

 which lodges the floccular lobe of the cerebellum. In some 

 skulls another wide and shallow but fairly prominent depres- 

 sion is seen dorsal to and slightly behind the floccular fossa, 

 this also lodges part of the cerebellum. Behind the internal 

 auditory meatus, between the periotic and exoccipital is seen 

 the internal opening of the foramen lacerum posterius 

 (fig. 72, 22). The shape of this opening varies. The ventro- 

 aiiterior border of the periotic is marked by a deep notch, the 

 sides of which sometimes unite, converting it into a foramen. 



On the outer -side of the periotic, and clearly seen only 

 after the removal of the tympanic, are two holes, the fenestra 

 ovalis and the fenestra rotunda. 



The tympanic (figs. 72, 15 and 75, 4) is a greatly expanded 

 boat-shaped bone, which forms the auditory bulla and lies 

 immediately ventral to the periotic ; it is separated from the 



1 The dura mater is a membrane which lines the cranial cavity and 

 is formed of tough connective tissue. 



