  Conditions on the elastic stiffnesses of anisotropic crystals are derived
such that circularly polarized longitudinal inhomogeneous plane waves with an
isotropic slowness bivector may propagate for any given direction of the normal
to the sagittal plane. Once this direction is chosen, then the wave speed, the
direction of propagation, and the direction of attenuation are expressed in
terms of the mass density, the elastic stiffnesses, and the angle between the
normal to the sagittal plane and the normals (also called "optic axes") to the
planes of central circular section of a certain ellipsoid. In the special case
where this angle is zero, and in this special case only, such waves cannot
propagate.
