  In the Skyrme model with massless pions, the minimal energy multi-Skyrmions
are shell-like, with the baryon density localized on the edges of a polyhedron
that is approximately spherical and generically of the fullerene-type. In this
paper we show that in the Skyrme model with massive pions these configurations
are unstable for sufficiently large baryon number. Using numerical simulations
of the full nonlinear field theory, we show that these structures collapse to
form qualitatively different stable Skyrmion solutions. These new Skyrmions
have a flat structure and display a clustering phenomenon into lower charge
components, particularly components of baryon numbers three and four. These new
qualitative features of Skyrmions with massive pions are encouraging in
comparison with the expectations based on real nuclei.
