Many of the fossil mollusks found in the Tertiary and Cretaceous rocks of Texas have been preserved in this manner. In some of the specimens the original shell material is so well preserved that the iridescent mother-of-pearl layer of the shell is found virtually intact. This type of preservation is less common, however, in the older rocks of the State. [Illustration: PLATE 2 Types of Fossil Preservation] Figures— 1. Internal mold of a Texas Cretaceous ammonite (×½). 2. Internal and external molds of gastropods and pelecypods in Cedar Park limestone member of the Walnut clay of Comanchean age (×½). Specimen from quarry near Cedar Park, Williamson County, Texas. 3. Internal mold of a Texas Cretaceous pelecypod (×½). 4. Fossil worm tubes on mold of a Cretaceous ammonite (×½). 5. Petrified or permineralized mammal bone of Tertiary age (×½). 6. Internal mold (steinkern) of a typical Texas Cretaceous gastropod (×½). 7. Carbon residue of a Tertiary fish (×¼). At certain localities in north and central Texas the Woodbine sands of Upper Cretaceous age (geologic time scale and geologic map, Pls. 1, 10) contain large numbers of shark and fish teeth (Pl. 37), fish scales and vertebrae. The remains of these vertebrates are unusually well preserved and are prized by both amateur and professional collectors. Calcareous Remains Hard parts composed of calcite (calcium carbonate) are very common among the invertebrates. This is particularly true of the shells of clams, snails, and corals. Many of these shells have been preserved with little or no evidence of physical change (Pl. 2). Phosphatic Remains The bones and teeth of vertebrates and the exoskeletons of many invertebrates contain large amounts of calcium phosphate. Because this compound is particularly weather resistant, many phosphatic remains (such as the fish teeth in the Woodbine sands) are found in an excellent state of preservation. Siliceous Remains Many organisms having skeletal elements composed of silica (silicon dioxide) have been preserved with little observable change. The siliceous hard parts of many microfossils and certain types of sponges have become fossilized in this manner (Pl. 14). Chitinous Remains Some organisms have an exoskeleton (outer body covering) composed of chitin, a material that is similar to finger nails. The fossilized chitinous exoskeletons of arthropods and other organisms are commonly preserved as thin films of carbon because of their chemical composition and method of burial. ALTERED HARD PARTS OF ORGANISMS