On a subvariety of the moduli space.
We give an explicit description of a non-normal irreducible subvariety of the moduli space of Riemann surfaces of genus 3 characterized by a non-cyclic group action. Defining equations of a family of curves representing non-normal points of this subvariety are computed. We also find defining equations of the family of hyperelliptic curves of genus 3 whose full automorphism group is C2 X C4. This completes the list of full automorphism groups of hyperelliptic curves.