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