Variation in θ = δΣ/Σ, the ratio of the perturbation surface density to the unperturbed surface density, with τ, dimensionless time in the sheared frame, plotted for a gravitationally-coupled two-fluid (stars and gas) system where both fluids have finite thickness. Here, Q_s = 2.0, Q_g = 2.0, and ϵ = 0.2. The effect of finite thickness prevails over the effect of the interstellar gas, even for this case with high gas-fraction, and hence the stellar disk can no longer host strong, small-scale spiral arms (for details see text). The net amplitude αθ ≪ 1 at all τ, where α is a scale factor.