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.