Abstract : It has long been recognized that Lg waves are not observed on paths traversing oceanic crust, but this has not yet been fully explained. Using normal mode analysis and finite difference simulations, we demonstrate that: (1) the overall thickness of the crustal waveguide affects the number of normal modes in a given frequency range; in general, thinner crust accommodates fewer modes; (2) 6-km thick oceanic crust does not allow Lg to develop as a significant phase in the frequency band 0.3-2 Hz due to the limited number of modes that exist; (3) in continental crust thicker than 15 km there are usually sufficient modes that Lg is stable; (4) the shallow sediment layer plays important roles in crustal guided wave propagation; trapping energy near the surface, separating Lg and Rg waves; (5) a 100-km long segment of oceanic structure on a mixed ocean/continent path can block P-SV type Lg propagation. The primary reason why Lg does not travel through oceanic crust thus lies in the structure of the crustal waveguide, with the decisive factor being the crustal thickness. The detailed shape of ocean to continent crustal transitions can influence Lg blockage, but the general inefficiency of Lg propagation in the oceanic structure is the dominant effect. Regional P and S waves decay with different rates due to complex geometric spreading and attenuation factors, hence discriminants based on P/S ratios are distance dependent. In a region without Lg blockage, the distance dependence, gamma, of Pg/Lg ratios and the factors influencing the distance dependence are of concern for the correction of the path effect. 80 earthquakes in the Western U.S. recorded at four stations of the Livermore NTS Network are used to determine the regional distance dependence of Pg/Lg ratios.