Morphogenesis of the intrahullar septum in Canis lupus Linnaeus, 1758 and Vulpes vulpes (Linnaeus, 1758), as well as the definitive septal morphology within the family, demonstrate that no hone of the auditory bulla bends its edge inwards the bulla to produce a septum. Instead, the canid intrabullar partition is a result of the development of the ventral entotympanic sinus, an additional cavity within the hypotympanic space. This sinus appears as a separate inflation of the ventral part of the caudal entotympanic and expands beneath the surrounding wall of the bulla. In this way the boundary of the sinus-forming inflation enters partially into the bulla cavity and becomes a partition consisting of two compositionally different components; the dorsal and ventral septa. The dorsal septum is exclusively caudal-entotympanic; the ventral septum is apparently bilaminar since it is formed by the growth of the caudal entotympanic over the ectotympanic. The only argument against the homology of canid and felid septa is a difference in their caudal-entotympanic regions participating in the septum-forming inflation. Currently available information about the canid septal patterns strongly suggests that it is the presence of the ventral entotympanic sinus that can be used as an auditory-bulla character uniquely shared by Canidae.
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