Abstract: A flexible, resilient, slit valve (10) including a flexible, resilient head (12) centered on a central axis (27) and extending laterally therefrom, an annular, peripheral attachment portion (19) centered on the central axis, and an annular, flexible, resilient, intermediate portion (20). The head has a convex interior surface (16) and a concave exterior surface (14). The intermediate portion (20) has an interior surface (26) an exterior surface (24). The exterior surfaces (14, 24) intersect at a circular line of intersection (32) having a diameter D1. The exterior surface (24) and the peripheral attachment portion (19) intersect at a circular line of intersection (34) having a diameter D2. The lines of intersection (32, 34) lie in parallel planes extending transverse to the central axis (27) and spaced from each other along the central axis by a distance L1. The ratio of D1/D2 is in the range of 0.600 to 0.900 and the ratio of D2/L1 is in the range of 14.000 to 19.