Part 36

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.

Theca—a sheath or case; in coelenterates, the bounding wall at or near the margin of the exoskeleton; in echinoderms, the main body skeleton (or calyx) which houses the animal’s soft parts; in graptolites, any cup or tube of the colony.

Thorax—in trilobites, that part of the body between the cephalon and pygidium.

Time-unit—a portion of continuous geologic time (e.g., eras, periods, epochs, and ages).

Time-rock unit—same as time-stratigraphic unit.

Time-stratigraphic unit—term given to rock units with boundaries established by geologic time; strata deposited during definite portions of geologic time (e.g., systems, series, stages, etc.).

Topography—the physical features or configuration of a land surface.

Topographic map—a map showing the physical features of an area, especially the relief and contour of the land.

Transverse—at right angles to length.

Triassic—the youngest period of the Mesozoic era; follows the Permian period of the Paleozoic and precedes the Jurassic period of the Mesozoic.

Trilobite—an extinct marine arthropod having a flattened segmented body covered by a hardened dorsal exoskeleton divided into three lobes.

Trivial name—the Latinized name added to a generic name to distinguish the species; same as specific name.

Type locality—the geographic location at which a formation was first described and from which it was named; or from which the type specimen of a fossil species comes.

Type specimen—the individual or specimen on which the original designation of a species was established.

Umbilicus—an external depression or opening at the center of many loosely coiled shells; in gastropods it is usually located at the base of the shell; in cephalopods it is usually located laterally.

Umbo—the arched part of the valve near the beak in bivalve shells.

Unicellular—composed of one cell.

Valve—the one or more pieces comprising the shell of animals.