x.] CARNIVORA. 171 



the septum and the promontory of the periotic, which 

 fissure expands posteriorly, or rather at its outer end, into a 

 triangular space, placed just over the fenestra rotunda, so 

 that the opening of this fenestra is partly in the outer and 

 partly in the inner chamber of the bulla. This chamber is 

 formed by a simple capsule of very thin but dense bone.- 

 deficient only at a small oval space in the roof, where the 

 periotic projects into and fills up the gap, except such 

 portion of it as is left to form the aperture of communica- 

 tion with the outer chamber. 



Not only are these two chambers thus distinct, but they 

 are originally developed in a totally different manner. At 

 birth the only ossification in the whole structure is the in- 

 complete ring of bone supporting the membrana tympani, 

 and developed originally in fibrous tissue. Ossification 

 extends from this, so as to complete the outer chamber, 

 and the very limited lip of the meatus auditorius externus. 

 The inner chamber is formed from a distinct piece of hya- 

 line cartilage, which at birth is a narrow slip, pointed at 

 each end, lying between the tympanic ring and the basi- 

 occipital, applied closely to the surface of the already 

 ossified periotic, and forming no distinct prominence on the 

 under surface of the skull. Soon after birth this increases 

 in size, and gradually assumes the bullate form of the wall 

 of the inner chamber. In young animals, even some time 

 after the ossification of the bulla is complete, the distinc- 

 tion between the two parts is clearly seen externally ; not 

 only are they marked off by a groove, but the tympanic 

 portion has a more opaque appearance than the other. The 

 septum is formed by an inversion of the walls of both ap- 

 plied together, and ultimately perfectly fused in Felts, although 

 remaining permanently distinct in some of the Virerrida. 



The carotid foramen in the TigQr is only represented by 



