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Groundwater conditions formed by water-bearing rocks (aquifers) in which the water is confined above and below by impermeable beds. These systems are named after the province of Artois in France, where artesian wells were first observed.Because the…

The largest phylum in the animal kingdom, including the well-known insects, spiders, ticks, and crustaceans, as well as many smaller, less well-known groups. Members of the phylum Arthropoda, that is, arthropods, are the dominant metazoans…

The only surviving subclass of the echinoderm class Crinoidea. The Articulata include all living members of the echinoderm class Crinoidea and are generally regarded as the post-Paleozoic crinoids. Crinoids are exclusively marine organisms, as are…

An order comprising the even-toed ungulates (hoofed mammals). There are two main radiations: the predominantly omnivorous Bunodontia, including suoids (such as pigs, peccaries, and hippos); and the more herbivorous Selenodontia, including camels and…

A group of highly fibrous, naturally occurring minerals with separable, long and thin fibers. Asbestos fibers are strong and flexible enough to be spun and woven as well as resistant to heat and chemicals. Historically, asbestos was used for fire…

An order of nematodes in which the oral opening is generally surrounded by three or six labia; in some taxa, labia are absent, but the cephalic sensilla are always evident. Usually there are eight cephalic or labial sensilla; the submedians may be…

A class of Tunicata (Urochordata) which occurs as solitary zooids or, by a process of asexual budding, develops into colonies. Zooids of the class Ascidiacea vary in length from a few millimeters to 10 in. (25 cm). Individuals or colonies are…

A subtaxon of marine crustaceans in the subclass Thecostraca. Members of the Ascothoracica are ectoparasites or endoparasites of cnidarians and echinoderms. The body is enclosed in a voluminous saclike mantle that is up to 20 mm (0.8 in.) in length;…

The largest of the world's continents. With its peninsular extension, commonly called the continent of Europe, it is the major portion of the broad east-west extent of the Northern Hemisphere landmasses. In many ways Asia is more a cultural concept…

A small group of trematodes (flukes) with about 80 freshwater and marine species in 4 families and 13 genera, infecting fishes (chondrichthyans and teleosts) and turtles. The Aspidogastrea vary in length from about 2 mm (0.08 in.) to several…

One of the many thousands of small planets (minor planets) revolving around the Sun, mainly between the orbits of Mars and Jupiter. The presence of a gap in J. A. Bode's empirical law of planetary spacings motivated a search for the missing planet.…

A layer of variable thickness within the upper mantle of the Earth that is mechanically weaker (less rigid) than both the overlying lithosphere and the underlying mesosphere. The depth of transition from lithosphere to asthenosphere can be as little…

An order of protozoans, subclass Holotrichia, in which all species are mouthless. All species are parasitic in other animals, typically oligochaete annelids. Many astomatids possess an elaborate holdfast organelle, of value in attachment to the…

The scientific discipline that studies the origin, evolution, distribution, and future of life in the universe. Astrobiology is an interdisciplinary science integrating contributions from biology, geology, astronomy, paleontology, and planetary…

That part of astronomy dealing with the position, distance, and motion of celestial objects, including solar system objects, stars, radio sources, and galaxies. New astrometry Astrometry has changed since the 1980s. Very long baseline interferometry…

Books, CD-ROMs, or computer-accessible sets showing stars, constellations, or other astronomical phenomena and their locations in the sky. Astronomical atlases provide two-dimensional views of the distributions of objects.The first sets of…

Lists or enumerations of astronomical data relevant to astronomy, navigation, geodesy, and space science applications. Astronomical catalogs vary a great deal in form and content depending upon the type of the data and the objects to which they are…

Schemes for locating astronomical objects in space. To an observer on the Earth's surface, the stars of the night sky appear to be placed upon a spherical shell of infinite radius with the observer at the center. Celestial objects appear to move…

The production of a permanent two-dimensional record of a scene of interest to astronomy. In direct imaging, the scene is a section of the sky optically imaged with a telescope. In other types of imaging, the scene is the output of an instrument,…

A telescope or telescopes, their protective enclosures (if any), support and headquarters buildings, and the staff of astronomers, engineers, technicians, and other support personnel. The telescopes can be optical or infrared (reflecting or…

The use of spectroscopy (the analysis of light as a function of wavelength) as a tool for obtaining observational data on the chemical compositions, physical conditions, and radial velocities of astronomical objects. Astronomical applications of…

A telescope adapted to the observation of the passage, or transit, of an astronomical object across the meridian of the observer. The astronomical transit instrument is the classic instrument of positional astronomy, which is the study of the…

The basic unit of length in the solar system. The astronomical unit (AU) is also used to a limited extent for interstellar distances through the definition of the parsec (1 pc = 206,265 AU). It is nearly equal to the mean distance a between the…

An order of actinopterygian (ray-finned) fishes consisting of only one family, Ateleopodidae, known as jellynose fishes. Ateleopodiformes along with the order Stomiiformes constitute a teleost superorder called the Stenopterygii. Jellynose fishes…

An order of Granuloreticulosia in which the naked amebas form branched, threadlike, interconnected pseudopodia (reticulopodia). Species are known from freshwater, salt water, and brackish water. General characteristics of the order Athalamida are…

An order of actinopterygian (ray-finned) fishes, including the silversides, rainbowfishes, and blue eyes. The members of the Atheriniformes, together with those of the Beloniformes and Cyprinodontiformes, comprise the series Atherinomorpha.…

The large body of seawater separating the continents of North and South America in the west from Europe and Africa in the east and extending south from the Arctic Ocean to the continent of Antarctica. The Atlantic is the second-largest ocean water…

A gaseous layer that envelops the Earth and most other planets in the solar system. Earth, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Titan (Saturn's largest satellite) are all known to possess substantial atmospheres that are held by the…

A scientific discipline concerned with the chemical composition of the Earth's atmosphere. Topics falling within the field include the emission, transport, and deposition of atmospheric chemical species; the rates and mechanisms of chemical…

An annular coral reef, with or without small islets, that surrounds a lagoon without projecting land area. Physiography Most atolls are isolated reefs that rise from the deep sea. They vary considerably in size and shape. Small rings, some without…

A device that uses an internal resonance frequency of atoms (or molecules) to measure the passage of time. The terms atomic clock and atomic frequency standard are often used interchangeably. A frequency standard generates pulses at regular…

Time that is based on physics, specifically on a unit of duration that corresponds to a defined number of periods of radiation of a specific atomic transition of a chosen isotope. The fundamental unit of atomic time is the International System…

A group of monoclinic calcic pyroxenes which have the general chemical formula (Ca,Mg,Fe)(Mg,Fe)-Si2O6, in which calcium is the dominant cation in the first cation position. Monoclinic pyroxene with substantial iron or magnesium in place of calcium…

An order of about 219 species of teleost fishes ranked in four suborders. The order Aulopiformes is distinguished by certain skeletal specializations of the gill arches that are unknown in other teleost fishes. They share characters with the order…

An optical manifestation of a large-scale electrical discharge process which surrounds the Earth. The discharge is powered by the so-called solar wind–magnetosphere generator. The Sun continuously blows out its upper atmosphere, the corona, with a…

An island continent situated in the Southern Hemisphere and extending from 10 to 44°S and from 113 to 153°E. Australia's total area (2,941,526 mi2 or 7,618,517 km2 for the Australian mainland and 26,383 mi2 or 68,332 km2 for the island of Tasmania)…

Minerals that are formed in sediment or a sedimentary rock. Their in-place origin distinguishes them from minerals that are formed elsewhere and transported to the site of deposition (detrital minerals). Authigenic minerals form at the Earth's…

In general, a large mass of snow, ice, rock, earth, or mud in rapid motion down a slope or over a precipice. In the English language, the term avalanche is reserved almost exclusively for snow avalanche. Minimal requirements for the occurrence of an…

The class of animals consisting of the birds. Birds (Fig. 1) are vertebrate animals comprising the class Aves. (In some classification schemes, the term Neornithes may also be used when referring to modern birds.) Primarily adapted for flying, the…

A prosimian primate in the family Daubentoniidae inhabiting Madagascar. The aye-aye (Daubentonia madagascariensis) [see illustration] is the most endangered of all mammals in Madagascar. Aye-ayes are members of the order Primates and the suborder…

A basic carbonate of copper with the chemical formula Cu3(OH)2(CO3)2. Azurite is normally associated with copper ores and often occurs with malachite. Azurite is monoclinic. It may be massive or may occur in tabular, prismatic, or equant crystals…

The common name for a number of carnivorous mammals in the family Mustelidae. Badgers are members of the carnivorous mammalian family Mustelidae, which also contains the weasels, otters, minks, ferrets, martens, sables, fishers, and wolverines.…

A semienclosed brackish sea located in a humic zone, with a positive water balance relative to the adjacent ocean (the North Sea and the North Atlantic). The Baltic is connected to the North Sea by the Great Belt (70% of the water exchange), the…

A sedimentary rock that was commonly deposited during the Precambrian. It was probably laid down as a colloidal iron-rich chemical precipitate, but in its present compacted form it consists typically of equal proportions of iron oxides (hematite or…

A mineral with chemical composition BaSO4. It is isostructural with celestite, SrSO4, and anglesite, PbSO4. Barite is orthorhombic, space group Pnma, a = 0.887 nanometer, b = 0.545 nm, c = 0.714 nm. The illustration shows typical crystals. Barite…

The common name for any of the perciform fishes belonging to a single genus, Sphyraena, in the family Sphyraenidae. Barracudas are characterized by an elongate, slightly compressed body; a long, pointed head; a large horizontal mouth with…

Elongate accumulations of sediment formed in the shallow coastal zone and separated from the mainland by some combination of coastal bays and their associated marshes and tidal flats. Barrier islands are typically several times longer than their…

An igneous rock characterized by small grain size (less than about 0.2 in. or 5 mm) and approximately equal proportions of calcium-rich plagioclase feldspar and calcium-rich pyroxene, with less than about 20% by volume of other minerals. Olivine,…

A low-lying area which is wholly or largely surrounded by higher land. An example is the Hudson Bay in northeastern Canada, which was formed by depression beneath the center of a continental ice sheet 18,000 years ago. Another example, the Qattara…

An order (or superorder) of the gastropod molluscan subclass Pulmonata containing about 2500 species that are grouped into 11–15 families. Only a few members of the family Ellobiidae are terrestrial, with the other species today being tidal to…

A common name applying to a large group of fishes, mostly in the order Perciformes, generally having the following characteristics: body about 2.5–3 times longer than its depth; two dorsal fins, with the first composed of stiff spines; pectoral fins…

An order of freshwater crustaceans (superorder Syncarida) whose members lack a carapace and whose head and thorax are not fused. All the thoracic appendages of bathynellaceans are biramous and similar. The sixth pleomere is fused to the telson,…

An elasmobranch subdivision of cartilaginous fishes known collectively as the rays, skates, or batoids. Batoidea is one of the two subdivisions of the subclass Elasmobranchii (class Chondrichthyes) and consists of 4 extant orders, 17–20 families,…

An order of actinopterygian (ray-finned) fishes consisting of a single family, Batrachoididae, collectively known as the toadfishes. The order Batrachoidiformes comprises a single family (Batrachoididae), four subfamilies, 23 genera, and…

Bauxite is the primary raw ore material used to produce aluminum metal and alumina (Al2O3). It is a naturally occurring weathering product composed of one or more aluminum hydroxide minerals, such as gibbsite [Al(OH)3], boehmite [AlO(OH)], and…

A class of the phylum Rotifera characterized by paired gonads and obligately asexual reproduction, found in diverse freshwater habitats worldwide. Bdelloid rotifers are among the most common microinvertebrates, particularly in ephemerally aquatic…

The bear family (Ursidae) contains the largest terrestrial carnivores. Most taxonomists agree that there are eight extant species of bears in three subfamilies: Ursinae (American black, Asiatic black, brown, polar, Malayan Sun, and sloth bears);…

The common name for the most primitive living rodent. The beaver has an ancestry that goes back to the time when modern mammals evolved. Two families and three species of beavers exist in the world. The family Aplodontidae contains a single species:…

The management and maintenance of colonies of honeybees for human purposes. Beekeeping is an ancient art. Although the commonly known honeybee species (Fig. 1) is native to Europe and Africa, humans have transported them to other continents. The…

An order of extinct 10-armed cephalopod mollusks with an internal shell. The order Belemnitida belongs to the subclass Coleoidea in the class Cephalopoda (phylum Mollusca). Constituent members, which are all extinct, are known as belemnites (Fig. 1)…

An order of actinopterygian (ray-finned) fishes containing the flyingfishes, needlefishes, and halfbeaks. Fishes in the order Beloniformes, along with those of the Atheriniformes and Cyprinodontiformes, comprise the series Atherinomorpha. Members of…

The term first applied to a particular, highly colloidal plastic clay found near Fort Benton in the Cretaceous beds of Wyoming. This clay swells to several times its original volume when placed in water and forms thixotropic gels when small amounts…

A water body north of the Pacific Ocean, 875,900 mi2 (2,268,000 km2) in area, bounded by the Siberian and Alaskan landmasses to the west and east respectively, and the Aleutian-Komandorskiye island arc to the south. The Bering Sea is very productive…

An order of the phylum Ctenophora (comb jellies) comprising two genera, Beroë and Neis. The single species of Neis (N. cordigera) is known only from Australia, but many species of Beroë have been described from polar to tropical waters all over the…

An order of somewhat intermediate position among actinopterygian (ray-finned) fishes. Members of the order Beryciformes share the following features with the perciform fishes: fin spines, ctenoid scales, an upper jaw bordered by the premaxillae, a…

The most common beryllium mineral. Beryl, Al2[Be3Si6O18], crystallizes in the hexagonal system with space group P6/mcc. The crystal structure consists of six-membered rings of corner-sharing silicon-oxygen (SiO4) tetrahedra cross-linked by…

Minerals containing beryllium as an essential component. Over 50 beryllium minerals have been identified, even though beryllium is a scarce element in the Earth's crust. The unusual combination of low charge (+2) and small ionic radius (0.035…

A cool and highly luminous star, prominently located in the right shoulder of the constellation Orion and noticeably red in color. Betelgeuse, or α Orionis, is a supergiant star about 130 parsecs (430 light-years) from the Sun. Its spectral type of…

An order of Zoomastigophorea (Protozoa). They are colorless, free-living cells, each with two flagella, one of which is used for attaching the organism to its exoskeleton (lorica). Although the attachment is normally from the front end, the…

Systems composed of two almost equal-size asteroids orbiting around their center of mass, or one or more satellites revolving around a larger primary body. The discovery of binary and multiple asteroids will aid the work of observers and theorists…

Two stars held together by their mutual gravitational attraction in a permanent (or long-term) association. The stellar universe is hierarchical. Stars exist singly and in binary pairs, multiple systems, and clusters. On large scales, roughly 105…

Marine sponges that are able to erode calcareous substrates such as coral, mollusk shells, and limestone, also called boring or excavating sponges. Confirmed eroders belong to the families Samidae, Thoosidae, Clionaidae, Spirastrellidae, Acarnidae,…

The study of the cycling of chemicals between organisms and the surface environment of the Earth. The chemicals either can be taken up by organisms and used for growth and synthesis of living matter or can be processed to obtain energy. The chemical…

A branch of meteorology and ecology that deals with the effects of weather and climate on plants, animals, and humans. Thermal equilibrium The principal problem for living organisms is maintaining an acceptable thermal equilibrium with their…

A member of a chemically diverse, structurally related group of minerals that comprise substantial fractions of both the Earth's crust and upper mantle. The term was coined by Albert Johannsen in 1911 to describe collectively the rock-forming…

The thin zone of living organisms and their environments at the surface of the Earth. The biosphere is the layer of life of the Earth (Fig. 1). Included in the biosphere are all environments capable of sustaining life above, on, and beneath the…

An iron-magnesium-rich layer silicate; it is also known as black mica. Biotite is the most abundant of the mica group of minerals. The name is derived from that of the French chemist J. Biot. The formula for the ideal end member, phlogopite, is…

The name for two species (genus Bison) of the cattle family Bovidae in the mammalian order Artiodactyla found in North America and Europe. The North American bison (Bison bison; see illustration) is often called buffalo, but should not be confused…

A term used to designate naturally occurring or pyrolytically obtained substances of dark to black color consisting almost entirely of carbon and hydrogen with very little oxygen, nitrogen, and sulfur. Bitumen may be of variable hardness and…

An extinct group of epifaunal bivalves that constructed reefs during the late Mesozoic. The valves of rudists are coiled to cylindrical in external form, and the shapes and sizes of rudist shells are extreme relative to other members of the class…

One of the seven existing classes in the phylum Mollusca; sometimes known as Pelecypoda. All members of the Bivalvia (bivalves) are aquatic, living at all depths of the sea and in brackish waters and freshwaters. With about 25,000 living species…

A region of spacetime exerting a gravitational field so strong that neither matter nor radiation can escape. Black holes are extreme cosmic objects predicted by German-born U.S. physicist Albert Einstein's theory of general relativity. Within a…

Researchers recently discovered a black hole with a mass estimated at 3.3 times the mass of the Sun, which is significantly less than the lowest-mass black holes previously detected with approximately 5 times the Sun's mass. The new finding opens…

A semienclosed marginal sea with an area of 420,000 km2 (160,000 mi2) bounded by Turkey to the south, Georgia to the east, Russia and Ukraine to the north, and Romania and Bulgaria to the west (Fig. 1). The physical and chemical structure of the…

A distinctive, fine-grained mudrock that derives its dark color from an unusually high content of organic matter, ranging from around 2% to over 50% of the weight of the rock (typical shales contain less than 1% organic matter). Most black shales…

A class of extinct Pelmatozoa in the subphylum Blastozoa which arose in the Ordovician and flourished in the Carboniferous and Permian. Blastoids did not survive the Permo-Triassic mass extinction event. Historically, two orders of the Blastoidea…