  Based on an extensive spectral study of a photometrically confirmed sample of
Mira variables, we find a relationship between relative Balmer emission-line
strength and spectral temperature of O-rich Mira stars. The $F_{\rm
H\delta}/F_{\rm H\gamma}$ flux ratio increases from less than unity to five as
stars cool down from M0 to M10, which is likely driven by increasing TiO
absorption above the deepest shock-emitting regions. We also discuss the
relationship between the equivalent widths of the Balmer emission lines and the
photometric luminosity phase of our Mira sample stars. Using our 291 Mira
spectra as templates for reference, 191 Mira candidates are newly identified
from the LAMOST DR4 catalog. We summarize the criteria adopted to select Mira
candidates based on emission-line indices and molecular absorption bands. This
enlarged spectral sample of Mira variables has the potential to contribute
significantly to our knowledge of the optical properties of Mira stars and will
facilitate further studies of these late-type, long-period variables.
