  In the (super)twistor formulation of massless (super)particle mechanics, the
mass-shell constraint is replaced by a "spin-shell" constraint from which the
spin content can be read off. We extend this formalism to massive
(super)particles (with N-extended spacetime supersymmetry) in three and four
space-time dimensions, explaining how the spin-shell constraints are related to
spin, and we use it to prove equivalence of the massive N=1 and BPS-saturated
N=2 superparticle actions. We also find the supertwistor form of the action for
"spinning particles" with N-extended worldline supersymmetry, massless in four
dimensions and massive in three dimensions, and we show how this simplifies
special features of the N=2 case.
