The propagation patterns of an incident quasi-longitudinal/transverse seismic wave in an intermediate monoclinic layer between highly anisotropic distinct triclinic half – spaces are analysed. Velocity profile of the incident/scattered seismic wave has been found analytically along with closed-form solutions for reflection/transmission coefficients. Velocity profiles for both the unconstrained and constrained geometries are plotted against propagation direction. The critical angle for both the incident quasi P/SV waves is computed and simulations are then performed to further study the effect of relevant parameters on phase velocities and coefficients for lithium niobate monoclinic material between triclinic media. Four published results on reflection/transmission phenomena of quasi P/SV waves are derived as particular cases to present study.