  We analyze the effect of interphase boundary anisotropy on the dynamics of
lamellar eutectic solidification fronts, in the limit that the lamellar spacing
varies slowly along the envelope of the front. In the isotropic case, it is
known that the spacing obeys a diffusion equation, which can be obtained
theoretically by making two assumptions: (i) the lamellae always grow normal to
the large-scale envelope of the front, and (ii) the Jackson-Hunt law that links
lamellar spacing and front temperature remains locally valid. For anisotropic
boundaries, we replace hypothesis (i) by the symmetric pattern approximation,
which has recently been found to yield good predictions for lamellar growth
direction in presence of interphase anisotropy. We obtain a generalized
Jackson-Hunt law for tilted lamellae, and an evolution equation for the
envelope of the front. The latter contains a propagative term if the initial
lamellar array is tilted with respect to the direction of the temperature
gradient. However, the propagation velocity of the propagative wave modes are
found to be small, so that the dynamics of the front can be reasonably
described by a diffusion equation with a diffusion coefficient that is modified
with respect to the isotropic case.
