  We report on the rheology of a lamellar lyotropic surfactant solution
(SDS/dodecane/pentanol/water), and identify a discontinuous transition between
two shear thinning regimes which correspond to the low stress lamellar phase
and the more viscous shear induced multi-lamellar vesicle, or ``onion'' phase.
We study in detail the flow curve, stress as a function of shear rate, during
the transition region, and present evidence that the region consists of a shear
banded phase where the material has macroscopically separated into bands of
lamellae and onions stacked in the vorticity direction. We infer very slow and
irregular transformations from lamellae to onions as the stress is increased
through the two phase region, and identify distinct events consistent with the
nucleation of small fractions of onions that coexist with sheared lamellae.
