THE dorsally placed external nostrils of plesiosaurs are usually regarded as an adaptation to breathing in those extinct marine reptiles. We suggest instead that the narial system was used in underwater olfaction. The internal nares are anterior to the external nares. Hydrodynamic pressure during swimming forced water into the mouth, along palatal grooves into the scoop-shaped internal nares and up short ducts, presumably lined with olfactory epithelia. Alternatively, or additionally, the so far unlocated Jacobson's organ detected particulate matter. The water was sucked out through the external nares by hydrodynamic pressures generated by fast flow over the convex upper surface of the head.