  A family of potential-density pairs that represent spherical shells with
finite thickness is obtained from the superposition of spheres with finite
radii. Other families of shells with infinite thickness with a central hole are
obtained by inversion transformations of spheres and of the finite shells. We
also present a family of double shells with finite thickness. All
potential-density pairs are analytical and can be stated in terms of elementary
functions. For the above-mentioned structures, we study the circular orbits of
test particles and their stability with respect to radial perturbations. All
examples presented are found to be stable. A particular isotropic form of a
metric in spherical coordinates is used to construct a General Relativistic
version of the Newtonian families of spheres and shells. The matter of these
structures is anisotropic, and the degree of anisotropy is a function of the
radius.
