  We report anomalous structure in the magnetoresistance of
SiO$_2$/Si(100)/SiO$_2$ quantum wells. When Landau levels of opposite valleys
are driven through coincidence at the Fermi level, the longitudinal resistance
displays elevations at filling factors that are integer multiples of 4
$(\nu=4i)$ accompanied by suppression on either side of $\nu=4i$. This persists
when either magnetic field or valley splitting is swept leading to resistance
ridges running along $\nu=4i$. The range of field over which they are observed
points to the role of spin degeneracy, which is directly confirmed by their
disappearance under in-plane magnetic field. The data suggest a new type of
many-body effect due to the combined degeneracy of valley and spin.
