Devon derives its name from Dumnonia, which, during the British Iron Age and Roman Britain, was the homeland of the Dumnonii Celts. The Anglo-Saxon settlement of Britain resulted in the partial assimilation of Dumnonia into the Kingdom of Wessex during the eighth and ninth centuries. The western boundary with Cornwall was set at the River Tamarby King Æthelstan in 936. Devon was constituted as a shire of the Kingdom of England thereafter.