Dictionary of Ichthyology  

oral valve = the flap attached just inside the jaws which stop water escaping from the mouth during exhalation, helping to maintain a unidirectional flow. Usually a valve is found just inside the ring of teeth in the jaw. Posterior valves may also be present. Also called a buccal valve.

From Kingsley's "Textbook of Vertebrate Zoology, 1906. av = anterior oral valves open, g = gills, o = oesophagus, pv = posterior valves. The anterior valves close at expiration and the posterior ones open. The oral cavity enlarges and contracts through the motion of the oral walls, in heavy black here. Photograph by Brian W. Coad.

Canadian Museum of Nature
www.briancoad.com