The postpulmonary septum (PPS) is a relatively thick sheet of connective tissue covering the inferior surface of the lungs in varanid lizards. The primary connection of the PPS is to the medial surface of the ribs; additional connections occur to the inferior midline of the dorsal vertebrae, the pericardium, and a direct (through loose connective tissue) link to the surface of the lung. The PPS effectively partitions the coelomic cavity into peritoneal and pleural cavities. Investigation demonstrates that the PPS is not capable of preventing displacement of the more caudal (peritoneal) viscera, which is displaced cranially during terrestrial locomotion; this cranial displacement could impinge on the tidal volume of the lungs. Kinematic analyses of terrestrial and aquatic locomotion in Varanus salvator document the different propulsive mechanics used during movement through these two media, and, most importantly, the marked reduction in lateral displacement of the trunk during swimming. These findings, when combined with previous studies of the cardiovascular and respiratory system of varanids performing terrestrial locomotion, suggest that mosasaurs had a versatile, effective respiratory system and were likely capable of both sustained swimming and prolonged submersion, such as during ambush foraging.