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. Environments have had more influence on the fauna than has geologic age, giving the Rustler fauna a striking resemblance to faunas of older Permian rocks which occupied similar biotopes. The Rustler fauna is large and varied, containing 35 species of invertebrates.

Rocks of Cretaceous age crop out along the western margins of the deeper playa basins and along the southern and southeastern margins of the Llano Estacado. The Trinity group, containing only the Paluxy sandstone; the Fredericksburg group, consisting of the Walnut, Comanche Peak, Edwards, and Kiamichi formations; and the Washita group, containing only a portion of the Duck Creek formation, have been identified in the Llano Estacado.

The 15-minute Agua Fria quadrangle in southwestern Brewster County, Texas, is arid, sparsely vegetated, and includes diverse topographic features that result chiefly from complex structure and variation in rock resistance to erosion. The mountainous and more complicated southern part of the area has suffered much deformation by igneous intrusions and faulting. The Comanche series is represented by the Devils River limestone, Grayson marl, and Buda limestone.