Segment—one of the parts into which a body naturally separates or is divided; for example, segments of arthropods or annelid worms. Septal—pertaining to the septum. Septum (plural, septa)—a dividing wall or partition; in fusulinids, a partition between chambers in the fusulinid shell; in corals, one of the radiating, longitudinal, calcareous plates located within the corallite; in cephalopods, the transverse partitions between the chambers. Series—the rocks formed during an epoch; the time-stratigraphic term next in rank below a system. Serrate—notched like a saw. Sessile—animal attached to the sea floor more or less permanently. Silica—an oxide of silicon (SiO₂). Siliceous—containing or pertaining to silica. Silicification—the process of combining or impregnating with silica. Silurian—the third oldest period of the Paleozoic era; follows the Ordovician, precedes the Devonian. Sinus—an elongate depression on brachiopod shells. Siphuncle—in cephalopods, the segmented horny or calcareous tube which extends from the protoconch to the living chamber. Slickensides—polished and grooved surfaces that are the result of two rock masses sliding past each other as in faulting. Solitary—living alone; not part of a colony. Species—one of the smaller natural divisions in classification. Specific name—see Trivial name. Spicule—a minute spike or dart, skeletal element in sponges and holothurians. Stratum (plural, strata)—a single bed or layer of rock. Strike—the direction of a real or imaginary line that is formed by the intersection of a bed or stratum with a horizontal plane; strike is perpendicular to the dip. Subconical—less than conical in shape; almost a cone. Suture—the line of junction between two parts; in crinoids, the line of junction between two plates; in gastropods, the line of junction of the whorls as seen on the exterior of the shell; in cephalopods, the line of junction between a septum and the shell wall. Symmetry—orderly arrangement of parts of an object with reference to lines, planes, or points. _Bilateral symmetry_—the symmetrical duplication of parts on each side of a vertical anterior-posterior plane. _Radial symmetry_—the symmetrical repetition of parts around a common vertical dorso-ventrally disposed axis. _Pentamerous symmetry_—symmetry arranged in a pattern of fives. System—the rocks formed during a period; the time-stratigraphic term next in rank above a series. Taxonomy—that branch of science that deals with classification, especially in relation to plants, animals, or fossils. Tertiary—the oldest period of the Cenozoic era; follows the Cretaceous period of the Mesozoic and precedes the Quaternary period of the Cenozoic. Test—the protective covering of some invertebrate animals.