10 THE NECTURUS 



Stapes and Columella 



This combination of elements fits in the fenestra vestibuli 

 of the inner ear. The opercular part is a flat concave 

 disc where it rests on the fenestra. The outer surface 

 bears the columella which extends dorsally and laterally, 

 joining the articulating projection already mentioned on 

 the ventral side of the squamosal. 



Prootic 



This is an irregular, cubical bone found between the 

 squamosal and the brain case. The inner surface forms 

 one of the walls of the inner ear. On the ventral side there 

 is a large canal, the mastoid foramen. Dorsally there is 

 a canal for the anterior semicircular canal. This canal 

 opens out by a large foramen on the center of the posterior 

 face. Use bristles as probes to determine the course of the 

 canals. The prootic articulates with the quadrate, palato- 

 pterygoid and squamosal on its outer face, and with the 

 parasphenoid and parietal on its inner. 



Opisthotic 



The opisthotic is located at the posterior angle of the 

 skull. It is somewhat triangular in outline viewed from its 

 dorsal surface, and oval from the anterior face. It forms 

 the posterior wall for the inner ear and contains two of its 

 canals. The horizontal canal enters its outer edge and 

 opens into the inner ear by a large depressed opening in 

 the center of the anterior face. On the ventral edge of 

 the inner face, the posterior canal enters the bone. Follow 

 these canals with bristles and find their course. It articu- 

 lates on the outer face with the squamosal, on the inner 

 with the parietal, on the central with the exoccipital and 

 on the ventral with the parasphenoid. 



