The auditory bulla is separated from the skull and composed of two compact and dense bones (the periotic and tympanic) referred to as the tympanoperiotic complex. This complex is located in a cavity in the middle ear, which, in the Mysticeti, is divided by a bony projection and compressed between the exoccipital and squamosal, but in the odontoceti, is large and completely surrounds the bulla (hence called "peribullar"), which is, therefore, not connected to the skull except in physeterids. In the Odontoceti, the cavity is filled with a dense foam in which the bulla hangs suspended in five or more sets of ligaments. The pterygoid and peribullar sinuses that form the cavity tend to be more developed in shallow water and riverine species than in pelagic Mysticeti. In Odontoceti, the composite auditory structure is thought to serve as an acoustic isolator, analogous to the lamellar construction found in the temporal bone in bats.