Atreb´ates, ancient inhabitants of that part of Gallia Belgica, afterwards called Artois. A colony of them settled in Britain, in a part of Berkshire and Oxfordshire.
At´rek, a river of Asia, forming the boundary between Persia and the Russian Transcaspian territory, and flowing into the Caspian; length 250 miles.
Atreus (at´rūs), in Greek mythology, a son of Pelops and Hippodamīa, and grandson of Tantălus. Atreus was the father of Agamemnon, according to Homer; other writers call him Agamemnon's grandfather. He succeeded Eurystheus, his father-in-law, as King of Mycēnæ, and in revenge for the seduction of his wife by his brother Thyestes gave a banquet at which the latter partook of the flesh of his own sons. Atreus was killed by Ægisthus, a son of Thyestes. The tragic events connected with this family furnished materials to some of the great Greek dramatists.
Atri (ancient, Hadria), an episcopal city in the province of Teramo, Italy, 8 miles from the Adriatic. It has an old (thirteenth century) Gothic cathedral, ruins of ancient Roman walls and buildings, and a palace of the Agraviva family, who were Dukes of Atri from 1398 to 1775. Pop. 14,043.
At´riplex. See Orache.
A´trium, the entrance-hall and most important apartment of a Roman house, generally ornamented with statues, pictures, and imagines or ancestral likenesses, which were portrait masks in wax kept in cases. The atrium formed the reception-room for visitors and clients. It was lighted by the compluvium, an opening in the roof, towards which the roof sloped so as to throw the rainwater into a cistern in the floor called the impluvium.
In zoology the term is applied to the large chamber or 'cloaca' into which the intestine opens in the Tunicata.
At´ropa, the nightshade genus of plants. See Belladonna.