The Tascotal Mesa quadrangle is in southeastern Presidio and southwestern Brewster counties, Trans Pecos Texas. Bandera Mesa and Tascotal Mesa are reached most easily from Marfa, 50 miles north, by the Marfa-Lajitas road which traverses the quadrangle from north to south.

The fossils described in this paper substantiate the Permian age of the Rustler formation. The Rustler fauna is largely molluscan and has a Whitehorse aspect. Four species of the Rustler fauna are similar to species from Russian and Indian faunas, suggesting possible correlations with Upper Permian rocks.

The Squaw Creek quadrangle is partly within the Llano region and partly within the Edwards Plateau province. The Edwards Plateau is continuous along the southern border of the quadrangle, but erosion has dissected it until long fingers extend northward, the most important being between Mormon and Squaw Creeks and east of Squaw Creek.