Eocene—the next to earliest of the Tertiary epochs, follows the Paleocene and precedes the Oligocene. Equivalved—right and left valves subequal and (except for hinge structures) comprising mirror images of each other. Evolution—a term applied to those methods or processes and to the sum of those processes whereby organisms change through successive generations. Exoskeleton—an external skeleton, or hard covering for the protection of soft parts, particularly among invertebrates. Fault—the displacement of rocks along a zone of fracture. Fauna—an assemblage of animals (living or fossil) living in a given place at a given time. Flank—the side or lateral portion of anything. Flora—an assemblage of plants (living or fossil) living in a given place at a given time. Fold—in brachiopods, a major rounded elevation of shell which affects both inner and outer shell surfaces. Foramen—in brachiopods, the opening in the pedicle valve near the beak where the pedicle extends through the shell. Foraminifer—a protozoan usually possessing a calcareous, perforated, chambered shell, but shell may be chitinous or agglutinated; a member of the order Foraminifera, phylum Protozoa. Formation—a rock unit useful for mapping and distinguished primarily on the basis of lithologic characters. Fossil—the remains or traces of organisms buried by natural causes and preserved in the earth’s crust. _Guide fossil_—a fossil which, because of its limited vertical but wide horizontal distribution, is of value as a guide or index to the age of the rocks in which it is found. Fossiliferous—containing fossilized organic remains. Fusulinid—a spindle-shaped foraminifer: test shaped like a grain of wheat. Gastrolith—highly polished well-rounded pebbles found associated with certain reptilian fossils; “stomach stones.” Gastropod—a terrestrial or aquatic invertebrate, typically possessing a single-valved, calcareous, coiled shell; member of class Gastropoda, phylum Mollusca: snails and slugs. Geologic age—the age of an object as stated in terms of geologic time (e.g., a Pennsylvanian fern, Cretaceous dinosaur). Geologic map—map showing distribution of rock outcrops, structural features, mineral deposits, etc. Geologic range—the known duration of an organism’s existence throughout geologic time (e.g., Cambrian to Recent for brachiopods). Glauconite—a greenish mineral commonly formed in marine environments and essentially a hydrous silicate of iron and potassium. Goniatite—an ammonoid cephalopod with suture composed of smooth saddles and simple angular lobes; member of subclass Ammonoidea. Graptolite—an extinct, marine, colonial organism with chitinous hard parts; believed to belong to subphylum Hemichordata of phylum Chordata. Guide fossil—see Fossil. Habitat—the physical environment in which an organism lives.