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A rigid and wave-resistant marine structure that stands above its surroundings. To navigators, a reef is any rocky structure that poses a threat to navigation. Marine biologists and geologists specify that reefs are constructed by organisms that…

A class of vertebrates composed of turtles, lizards, snakes, crocodilians, amphisbaenians, and tuataras, as well as a variety of extinct forms. The members of the vertebrate class Reptilia are tetrapods, meaning that they possess four limbs or are…

An order of lepidosaurian reptiles represented by one extant species, Sphenodon punctatus, commonly known as the tuatara. The reptilian members of the order Rhynchocephalia are referred to as the tuatara (also tuataras) [Fig. 1]. There is one…

The dominant order of living reptiles, composed of the lizards, snakes, and amphisbaenians. The order Squamata, whose members are known as squamates, is the largest and most diverse order of reptiles. Squamates first appeared in Jurassic times and…

A diverse group of early jawed fishes from the Paleozoic Era, usually classified as a distinct class of vertebrates. Most acanthodians have tiny body scales with a cross section like that of an onion, the layers reflecting incremental scale growth.…

An order of actinopterygian (ray-finned) fishes represented by the sturgeons and paddlefishes. The order Acipenseriformes, the only extant order of about 10–12 orders of chondrosteans (subclass Chondrostei), probably originated from palaeonisciform…

An order of inarticulated brachiopods. Together with Lingulida and Siphonotretida, Acrotretida is one of the three orders of inarticulated brachiopods currently classified in class Lingulata in subphylum Linguliformea (phylum Brachiopoda).…

A class of teleostom fishes commonly known as the ray-finned fishes, which are distinguished by the structure of the paired fins that are supported by dermal rays. The class Actinopterygii comprises the subclasses Cladistia [Polypteriformes (bichirs…

A discipline that combines the means to know the location and heading of an aircraft with respect to some map or coordinate system, with guidance which means steering the aircraft along some desired path toward a destination. The construction of the…

A service to promote the safe, orderly, and expeditious flow of air traffic. Safety is principally a matter of preventing collisions with other aircraft, obstructions, and the ground; assisting aircraft in avoiding hazardous weather; assuring that…