A detailed discussion of Newtonian and general relativistic spherically sym
metric dust solutions leads to the following suggested criteria for a singu
larity to be classified as a shell-cross: (I) All Jacobi fields have finite
limits (in an orthonormal parallel propagated frame) as they approach the
singularity. (2) The boundary region forms an essential C-2 singularity whi
ch is C-1 regular, that is it can be transformed away by a C-1 coordinate t
ransformation.