Molds and casts are to be found in almost all of the fossil-bearing rocks of Texas, and they make up a large part of most fossil collections. It is particularly common to find fossil clams and snails preserved by this method. This is primarily because their shells are composed of minerals that are relatively easy to dissolve, and the original shell material is often destroyed. [Illustration: PLATE 3 Silicified Brachiopods All specimens from Permian limestones of the Glass Mountains, Brewster County, Texas] Figures— 1, 2. _Avonia_ sp., ×2. Ventral and side view of two pedicle valves showing long slender spines. 3. _Avonia_ sp., ×6. Young specimen showing attachment ring at apex. 4-6. _Muirwoodia multistriatus_ Meek, ×4. Respectively, side and ventral view of pedicle valve and dorsal view of brachial valve. 7-9. _“Marginifera” opima_ Girty. Respectively, ventral and side view of pedicle valve showing long stout spines (×4) and interior of brachial valve showing muscle scars and brachial ridges (×2). 10-13. _Aulosteges tuberculatus_ R. E. King, ×4. Respectively, side and interior view of brachial valve showing muscle scars; ventral view of pedicle valve showing brush of attachment spines on ears; and ventral view of a young pedicle valve. 14. _Avonia_ sp., ×4. Ventral view of a specimen with long spines. 15, 16. _Avonia subhorrida_ (Meek), ×2. Ventral view of a pedicle valve and dorsal view of a brachial valve showing spines on both. 17. _Avonia signata_ (Girty), ×2. Dorsal view of a large specimen showing hairlike spines on brachial valve. 18-20. _Prorichthofenia permiana_ (Shumard). Respectively, side and posterior view of pedicle valve (×4) and interior of dorsal valve (×2) showing anchor spines and interior spines of the brachial valve. 21. _Heteralosia hystricula_ (Girty), ×2. Cluster of individuals attached to a large _Marginifera_. Photograph courtesy of Dr. G. A. Cooper, U. S. National Museum. Tracks, Trails, and Burrows Many animals have left records of their movements over dry land or the sea bottom. Some of these, such as footprints (Pl. 4), indicate not only the type of animal that left them but often provide valuable information about the animal’s environment. Thus, the study of a series of dinosaur tracks would not only indicate the size and shape of the foot but also provide some information as to the weight and length of the animal. In addition, the type of rock containing the track would help determine the conditions under which the dinosaur lived.