Before revision:  We use scaling to collapse time dependent structure factors at multiple wavenumbers and find the effective dynamic exponent z_{ eff } \approx 3 near the critical point. This value and the form of the structure factor are in excellent agreement with recent theoretical predictions invoking hydrodynamic coupling to the bulk. Our result may have biological relevance since membranes isolated from cells are near miscibility critical points .
After revision:  We use scaling to collapse time dependent structure factors at multiple wavenumbers and find the effective dynamic exponent z_{ eff } \approx 3 as the correlation length of fluctuations exceeds a hydrodynamic length set by the membrane and bulk viscosities .