GeoWord of the Day

The GeoWord of the Day is a free service of the American Geosciences Institute. All of the terms and definitions are from the Glossary of Geology, 5th Edition Revised.

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trail [paleont] . (a) A fossil structure consisting of a trace or sign of the passing of one or many animals; esp. a more or less continuous marking left by an organism moving over the bottom, such as a worm trail. Cf: track [paleont]. (b) The extension of either valve of a brachiopod shell anterior to the geniculation (or anterior to the "visceral disc" or the part of the shell posterior to the geniculation) (TIP, 1965, pt.H, p.154, 155).

gyroid (gy'-roid). An isometric crystal form consisting of 24 crystal faces with indices { hkl } and symmetry 432. A gyroidal crystal may be right- or left-handed.

float coccolith . A modified coccolith serving as a suspension organ in nonmotile coccolithophores exhibiting dimorphism (such as Thorosphaera ).

living chamber (liv'-ing). (a) The body chamber housing the soft parts of a cephalopod. (b) The outermost part of the zooidal body cavity in stenolaemate bryozoans, which contains the functional organs.

inner side . The portion of a conodont element on the concave side of the anterior-posterior midline. Ant: outer side.

cladome (clad'-ome). The group of similar rays of a diaene, triaene, or tetraene sponge spicule.

stack [coast] . An isolated pillar-like rocky island or mass near a cliffy shore, detached from a headland by wave erosion assisted by weathering; esp. one showing columnar structure and roughly horizontal stratification. Examples occur off the chalk cliffs of the Normandy coast. A stack may also form along the shore of a large lake. Cf: skerry; chimney [coast]. Syn: marine stack; rauk; rock stack; sea stack.

invariant point . A point representing the conditions of invariant equilibrium.

eumalacostracan (eu''-mal-a-cos'-tra-can). A shrimplike crustacean belonging to the subclass Eumalacostraca, differing mainly from other malacostracans by the nonbivalve nature of the carapace and the presence of biramous thoracic appendages with a single joint in the protopod. Range, Middle Devonian to Holocene.

Scaldisian (Scal-dis'-ian). North Sea basin stage: lower Pliocene, in some usage also middle Pliocene (above Hemmoorian, below Amstelian); based on warm-climate mollusk faunas.