ARSACIDAe, the dynasty of Parthian kings, so called from the name of the founder, Arsaces. Very little is known of the circumstances attending the sudden rise of the Parthian power, or of the leader under whom it was effected. He is said by some to have been a Parthian noble, by others to have been a predatory Scythian chief. But at all events, about 250 b.c., when the Seleucid empire of Antiochus II. was distracted by an Egyptian war and the successful rebellion of Bactria, tùe Parthians, hitherto subject and almost unknown to history, revolted, established an independent kingdom, and made their leader, Arsaces, their first sovereign. He is said to have been killed in battle, after a short reign of three years, and the throne was then occupied by his brother, Tiridates, who, like all succeeding sovereigns of Parthia, assumed the name Arsaces as a regal title. The empire increased rapidly in extent and strength, and for a time was the most formidable rival of the Roman power. Towards the close of the 2d century a.d. it had fallen somewhat into decay, as is manifest from the successful Roman invasions under Avidius Cassius and the Emperor Severus; and it finally succumbed to the Persians, who (224 a.d.) revolted under Artaxerxes, and slew in battle A.rtabanus, the twentyninth and last of the Arsacidae. See Parthia.