For irrotational dust the shear tensor is consistently diagonalizable
with its covariant time derivative: sigma(ab) = 0 = sigma(ab), a not e
qual b, if and only if the divergence of the magnetic part of the Weyl
tensor vanishes: divH = 0. We show here that in that case, the consis
tency of the Ricci constraints requires that the magnetic part of the
Weyl tensor itself vanishes: H-ab = 0.