  We point out that extended gravity theories, the Lagrangian of which is an
arbitrary function of scalar curvature $R$, are equivalent to a class of the
scalar tensor theories of gravity. The corresponding gravity theory is
$\omega=0$ Brans-Dicke gravity with a potential for the Brans-Dicke scalar
field, which is not compatible with solar system experiments if the field is
very light: the case when such modifications are important recently.
