A flexible, resilient, slit valve to allow selective dispensing of a fluent product from an interior environment to an exterior environment, the valve comprising: a flexible, resilient head centered on a central axis and extending laterally therefrom, the head having: a convex interior surface to face an interior environment, a concave exterior surface to face an exterior environment, at least one self-sealing slit through the head, confronting, openable portions along the at least one slit to define a normally closed orifice in an unconstrained condition wherein the openable portions can move in a first direction toward the exterior environment to an open orifice configuration and return in an opposite direction to a closed configuration, and a laterally outwardly facing peripheral surface extending from the interior surface toward the exterior surface, the peripheral surface centered on the central axis; an annular, peripheral attachment portion centered on the central axis and spaced laterally from the head; and an annular, flexible, resilient, intermediate portion centered on the central axis and extending laterally from the head to the peripheral attachment portion, the intermediate portion having: an interior surface to face said interior environment, and an exterior surface to face said exterior environment; and wherein; the exterior surfaces (14, 24) of the head and the intermediate portion intersect at a first circular line of intersection having a diameter D 1, the exterior surface of the intermediate portion and the peripheral attachment portion intersect at a second circular line of intersection having a diameter D 2, the second circular line of intersection is spaced in the first direction from the first circular line of intersection, the first and second circular lines of intersection (32, 34) lie in parallel planes extending transverse to the central axis and spaced from each other along the central axis by a distance L 1, characterised in that the ratio of D 1 /D 2 is in the range of 0,600 to 0,900 in an as-molded condition of the valve, and the ratio of D 2 /L 1 is in the range of 14,000 to 19,000 in said as-molded condition of the valve. a flexible, resilient head centered on a central axis and extending laterally therefrom, the head having: a convex interior surface to face an interior environment, a concave exterior surface to face an exterior environment, at least one self-sealing slit through the head, confronting, openable portions along the at least one slit to define a normally closed orifice in an unconstrained condition wherein the openable portions can move in a first direction toward the exterior environment to an open orifice configuration and return in an opposite direction to a closed configuration, and a laterally outwardly facing peripheral surface extending from the interior surface toward the exterior surface, the peripheral surface centered on the central axis; a convex interior surface to face an interior environment, a concave exterior surface to face an exterior environment, at least one self-sealing slit through the head, confronting, openable portions along the at least one slit to define a normally closed orifice in an unconstrained condition wherein the openable portions can move in a first direction toward the exterior environment to an open orifice configuration and return in an opposite direction to a closed configuration, and a laterally outwardly facing peripheral surface extending from the interior surface toward the exterior surface, the peripheral surface centered on the central axis; an annular, peripheral attachment portion centered on the central axis and spaced laterally from the head; and an annular, flexible, resilient, intermediate portion centered on the central axis and extending laterally from the head to the peripheral attachment portion, the intermediate portion having: an interior surface to face said interior environment, and an exterior surface to face said exterior environment; and an interior surface to face said interior environment, and an exterior surface to face said exterior environment; and the exterior surfaces (14, 24) of the head and the intermediate portion intersect at a first circular line of intersection having a diameter D 1, the exterior surface of the intermediate portion and the peripheral attachment portion intersect at a second circular line of intersection having a diameter D 2, the second circular line of intersection is spaced in the first direction from the first circular line of intersection, the first and second circular lines of intersection (32, 34) lie in parallel planes extending transverse to the central axis and spaced from each other along the central axis by a distance L 1, characterised in that the ratio of D 1 /D 2 is in the range of 0,600 to 0,900 in an as-molded condition of the valve, and the ratio of D 2 /L 1 is in the range of 14,000 to 19,000 in said as-molded condition of the valve.