  Planar polynomial automorphisms are polynomial maps of the plane whose
inverse is also a polynomial map. A map is reversible if it is conjugate to its
inverse. Here we obtain a normal form for automorphisms that are reversible by
an involution that is also in the group of polynomial automorphisms. This form
is a composition of a sequence of generalized Henon maps together with two
simple involutions. We show that the coefficients in the normal form are unique
up to finitely many choices.
