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Results from coincident observations with IUE and the McMath--Pierce telescope are presented for the asymptotic giant branch (AGB) stars R~Leo (M8~IIIe, M) and R~Lyr (M5~III, SRb). This survey was made to investigate the differences between the atmospheric structure of Mira--type and semiregular variables through the Fe~I (42) lines at 4202~\AA\ and 4308~\AA. These lines are fluoresced by the Mg~II lines near 2800~\AA\ via an Fe~I (UV3) transition. The Fe~I (42) lines peak in flux at the same phase ($\sim$0.3) as the Mg~II lines, which corresponds to a $\sim$0.3 phase lag behind the peak Balmer line flux. NLTE radiative transfer calculations show that this phase lag between the Mg~II and the Balmer line peak fluxes results from the existence of a permanent chromosphere in the Mira. The fact that Fe~I (42) variability matches the Mg~II variability indicates that these fluorescent lines originate from a cooler circumstellar shell and \underline{not} from the inner--shock region or photosphere. The Fe~I (42) lines in the semiregular variable do not vary from changes in the Mg~II flux. This suggests that the circumstellar shells surrounding semiregulars are not as optically thick as those around Miras.
