Gastropod shells, both Recent and fossil, exhibit a great variety of size, shape, and ornamentation. Such shells may be cone-shaped, spirally coiled, flat, turreted, or cylindrical. The shell is commonly wound in a spiral around a central axial pillar (the _columella_). The closed pointed end of the shell is called the _apex_, and each turn of the shell is called a _whorl_ (fig. 16). The last-formed and largest whorl is called the _body whorl_, and this whorl contains the _aperture_—the opening of the shell. The combined whorls exclusive of the body whorl are known as the _spire_. The inner and outer margins of the aperture are designated the _inner lip_ and the _outer lip_, respectively. In some snails the aperture is closed by means of the _operculum_—a calcareous or horny plate attached to the foot of the animal. This plate effectively seals the aperture when the animal is withdrawn into its shell. Some gastropods have shells that are loosely coiled, and in these forms the columella is absent. If the whorls of such shells are not in contact on the inner surface, this leaves an open space which is called the _umbilicus_ (fig. 16a). The umbilicus is commonly seen as an opening in the base of the gastropod shell, but in some forms the umbilical opening may be partially or completely covered by a thick growth of shell called the _callus_. Many gastropods, particularly those of the Texas Cretaceous, are commonly preserved as internal or external molds. This type of preservation occurs after the death of the animal, and the decomposition of the soft parts enables the shell to become filled with sediment. This filling later becomes solidified, and the outer shell may eventually be removed by weathering or solution. This type of internal mold is called a _steinkern_ and normally does not reflect any external shell characteristics (Pl. 2). In some of the Pennsylvanian and Tertiary formations, however, gastropods may be collected with the original shell in an excellent state of preservation. Plates 20-23 illustrate some typical Paleozoic, Mesozoic, and Cenozoic gastropods. CLASS PELECYPODA.— The pelecypods possess a shell composed of two calcareous valves (fig. 17) which enclose the soft parts of the animal. Members of this class live exclusively in an aquatic habitat and are most abundant in marine environments. Most pelecypods are slow-moving bottom-dwelling forms, but some, like the oysters, are attached. Still others, for example, the scallop or _Pecten_, are swimmers. The Pelecypoda include such familiar saltwater forms as the clams and oysters, as well as the common fresh-water mussel. Pelecypods range from Cambrian to Recent in age but are more abundant in Mesozoic and Cenozoic rocks.