Orientifold points in M theory

We identify the lift to M theory of the four types of orientifold points, a nd show that they involve a chiral fermion on an orbifold fixed circle. Fro m this lift, we compute the number of normalizable ground states for the SO (N) and Sp(N) supersymmetric quantum mechanics with sixteen supercharges. T he results agree with known results obtained by the mass deformation method . The mass of the orientifold is identified with the Casimir energy.