Abstract: A duckbill valve is provided based on a valve seal with a hollow cylindrical body with first and second ends, the first end having an open mouth, the second end having a closable mouth circumscribed by a pair of crescent shaped lips. One of the lips has a thinner width than the other. The cylindrical body flares outward in the direction from the first end towards the second end. In another aspect, the cylindrical body includes a posterior and an anterior section adjacent the respective first and second ends, the anterior section having an outer wall portion curvilinearly tapering toward the thinner lip. Valves of the present invention are particularly suitable for controlling flow of semi-solid substances because the mouth of the duckbill can open to a relatively large extent, the latter being off-center within a round collar fitting.