148 The Alligator and Its Allies 



in the Aves and Mammalia: the external auditory 

 meatus, the tympanic cavity, and the labyrinth. 

 It is the presence of the meatus that lifts the Croco- 

 dilia above the other Reptilia. 



Two strong folds of integument, one above and 

 one below, completely cover the outer ear and 

 allow it to open as a mere slit on the lateral surface 

 of the head a little back of the corner of the 

 eye. By lifting the upper valve one may perceive 

 the lower half of the meatus and the bottom of 

 the tympanic membrane. The upper valve is the 

 larger and is sickle shaped; the lower is smaller 

 and more three cornered. Both spring from the 

 outer surface of the squamosal bone, from its 

 posterior obtuse angle to its anterior union with 

 the postfrontal. The lower fold is raised highest 

 behind the corner of the eye and is lost in the middle 

 of the rima auditoria; by this Hasse indicates the 

 position of the outer opening of the external audi- 

 tory meatus. The form of the meatus may be 

 compared to a wedge whose base is directed dorso- 

 medio-caudad and whose edge points in a ventro- 

 latero-cephalic direction; its side walls are either 

 soft or bony ; its outer end is covered by the folds ; 

 at its inner end is the tympanic membrane or drum. 



The drum is a round, soft, elastic membrane in 

 which a radial arrangement of its constituent 

 fibers may be seen. It is funnel shaped from with- 

 out and above, and the fibers radiate from the apex 

 to which the columella is attached. The membrane 



