  The matrix theory description of the discrete light cone quantization of $M$
theory on a $T^{2}$ is studied. In terms of its super Yang- Mills description,
we identify symmetries of the equations of motion corresponding to independent
rescalings of one of the world sheet light cone coordinates, which show how the
$S$ duality of Type IIB string theory is realized as a Nahm-type
transformation. In the $M$ theory description this corresponds to a simple
$9-11$ flip.
