  Combining results of T.K. Lam and J. Stembridge, the type $C$ Stanley
symmetric function $F_w^C(\mathbf{x})$, indexed by an element $w$ in the type
$C$ Coxeter group, has a nonnegative integer expansion in terms of Schur
functions. We provide a crystal theoretic explanation of this fact and give an
explicit combinatorial description of the coefficients in the Schur expansion
in terms of highest weight crystal elements.
