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. Variety—a subdivision of a species, designated by a third name when a variety is designated. Ventral—pertaining to the abdomen; as opposed to dorsal, pertaining to the back. Vertebrate—an animal having a backbone or spinal column. Whorl—a single turn or volution of a coiled shell. -zoic—combining form meaning “life” (Greek _zoikos_, life). Zooecium (plural, zooecia)—tube or chamber occupied by an individual of the bryozoan colony; also called an autopore. Footnotes [1]Associate Professor of Geology, Lamar State College of Technology, Beaumont, Texas. [2]Entries marked with asterisk are published by the Bureau of Economic Geology, The University of Texas, Austin. Those not out of print are distributed at nominal sale price; list sent on request. These publications may be consulted at many public libraries and/or Chamber of Commerce offices. Index Page numbers in italics indicate illustrations.