An analogue of U-rank for atomic classes

For a countable, complete, first-order theory $T$, we study $At$, the class of atomic models of $T$. We develop an analogue of $U$-rank and prove two results. On one hand, if some tp(d/a) is not ranked, then there are $2^{{\mathrm{\aleph }}_1}$ non-isomorphic models in $At$ of size ${\mathrm{\aleph }}_1$. On the other hand, if all types have finite rank, then the rank is fully additive and every finite tuple is dominated by an independent set of realizations of pseudo-minimal types.