BAL^NOPTERA. 



43 



The inner surface, moderately convex, is marked with 

 striae, which deepen as they turn into the cavity. 

 The lower aspect has a strong keel, which is defined 

 externally by a shallow depression extending parallel 

 to it. The cavity opens at the thick, rounded, hori- 

 zontal upper border of the inner surface 2| inches 

 above keel ; Eustachian end faintly notched. 



Periotic, smooth on its lower surface, is 3 inches long 

 by 2 inches broad ; internally are the auditory and 

 other foramina ; externally it forms the inner wall of 

 the tympanic cavity. The malleus is attached to the 

 base of the lip-like process. Outer and inner surfaces 

 of the tjnnpanic bone figured below. 



