  The surface-impedance matrix method is used to study interfacial waves
polarized in a plane of symmetry of anisotropic elastic materials. Although the
corresponding Stroh polynomial is a quartic, it turns out to be analytically
solvable in quite a simple manner. A specific application of the result
concerns the calculation of the speed of a Stoneley wave, polarized in the
common symmetry plane of two rigidly bonded anisotropic solids. The
corresponding algorithm is robust, easy to implement, and gives directly the
speed (when the wave exists) for any orientation of the interface plane, normal
to the common symmetry plane. Through the examples of the couples
(Aluminum)-(Tungsten) and (Carbon/epoxy)-(Douglas pine), some general features
of a Stoneley wave speed are verified: the wave does not always exist; it is
faster than the slowest Rayleigh wave associated with the separated
half-spaces.
