  We discuss an ansatz for Skyrme fields in three dimensions which uses
rational maps between Riemann spheres, and produces shell-like structures of
polyhedral form. Houghton, Manton and Sutcliffe showed that a single rational
map gives good approximations to the minimal energy Skyrmions up to baryon
number of order ten. We show how the method can be generalised by using two or
more rational maps to give a double-shell or multi-shell structure.
Particularly interesting examples occur at baryon numbers twelve and fourteen.
