They are found in the southern United States and Mexico: H. theologus is mostly found west of the Colorado River, while H. selenopoides is mostly found to the east, with some populations in Death Valley and near Mercury, Nevada.
The text describes the geographic distribution of two species, H. theologus and H. selenopoides, highlighting their presence in the southern United States and Mexico.  H. theologus is more prevalent west of the Colorado River, while H. selenopoides is found to the east with some populations in Death Valley and near Mercury, Nevada.