222 Mr. D. M. S. Watson on the 



The vestibule lies very low down in the skull, being largely 

 below the bottom of the brain. It is considerably longer 

 than high and wide, its small posterior end being produced 

 backwards and outwards, separated by a thin ridge from the 

 deep groove which transmitted the IX., X,, and XI. nerves. 



The anterior end is continued as a rather large cavity, the 

 lateral wall of which bears a diverticulum for the ampulla of 

 the horizontal semicircular canal ; still further forward is the 

 space for the ampulla of the anterior vertical semicircular 

 canal. 



The vestibule is continued up by a special narrow part, 

 which passes into the joint portion of the anterior and poste- 

 rior semicircular canals. Nythosaurus shows all three 

 canals very well. The anterior canal loops over the fossa 

 subarcuata in the normal way, and the chief point of interest 

 seems to be that the ampullae are very small. 



The ductus endolymphaticus has a small foramen to the 

 cranial cavity, which lies just above and behind the fossa 

 subarcuata. 



The fenestra ovale lies in the outer and lower wall of the 

 vestibule, and the posterior end of the vestibular space opens 

 by a very large foramen (confluent with the foramen jugu- 

 laris, from which it is marked off by a distinct process) into 

 the deep pit into which all the posterior cranial nerves open. 

 This large opening is the fenestra rotundata ; immediately 

 to the front of it the vestibular space is continued down- 

 wards, forwards, and inwards into a long cavum cochlearum. 

 The cochlea was obviously quite long and bent perhaps 

 through a quadrant. 



The ear that has been just described resembles that of a 

 mammal, and differs from those of all other types in the 

 following features : — 



1. Its inferior position, the vestibule lying very largely 



below the brain, a character common to all Therapsids 

 in which the condition is known. 



2. The small fenestra ovale. 



3. The long, anteriorly internally directed, and bent 



cochlea. 



It is surprisingly primitive in the retension of a large 

 opening to the brain-cavity, closed, of course, in life by a 

 membrane. 



The semicircular canals also are of a very simple type, 

 without any of the specialization so often found in lizards, 

 birds, and mammals. 



