74 The Alligator and Its Allies 
ing which forms a communication through the roof 
of the brain case with the tympanic cavity of the 
other side. On its posterior wall is the prominent 
foramen through which the facial nerve passes on 
its way to its final exit from the skull through the 
exoccipital; this foramen is bounded by the quad- 
rate, squamosal, and exoccipital. The opening of 
the fenestra ovalis is in the fresh skull occupied by 
the expanded end of the auditory ossicle, the 
columella, whose outer end articulates by a con- 
cave facet with a trifid extracolumellar cartilage 
which reaches the tympanic membrane. The 
lower process of this extracolumella passes into a 
cartilaginous rod which lies in a canal in the 
quadrateand is during life continuous with Meckel’s 
cartilage within the articular bone of the mandible. 
“The columella and extracolumella are together 
homologous with the chain of mammalian auditory 
ossicles.” 
The Lower Jaw (Figs. 21, 23, and 24). The 
mandible consists of two similar rami, rather closely 
united at the anterior-median symphysis with 
each other. Each ramus consists of six bones. 
The dentary (Figs. 23 and 24, 18; Fig. 21, 20) is 
a long bone that unites at the symphysis with its 
fellow to form the point of the jaw. It bears, along 
its dorsal edge, about twenty teeth; all but the 
posterior four or five of these teeth are in individual 
sockets; this may vary somewhat with age. The 
outer surface of the dentary, especially towards the 
