A 



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 



