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 
