A systematic study has been made of the ionization cross sections for ion excitation of the $M$-shell x rays of elements with $Z$ in the region $72\ensuremath{\le}Z\ensuremath{\le}92$. Protons, $\mathrm{H}_{2}^{}{}_{}{}^{+}$ ions, and $\ensuremath{\alpha}$ particles with incident energies ranging from 60 to 400 keV were used to excite the $M$ lines of Hf, Ta, W, Au, Bi, and U. The $Z$ dependence of the proton-excited x-ray yield for these elements, although differing sharply from previously published $M$-shell results, is shown to be consistent with the $Z$ dependence for proton excitation of the $K$ and $L$ shells reported in earlier papers. Finally, we have compared the ionization cross sections of $\mathrm{H}_{2}^{}{}_{}{}^{+}$ ions at incident energy ${E}_{0}$ with those for protons at incident energy $\frac{1}{2}{E}_{0}$, and found that within 30% they are in the ratio 2: 1.