James Douglas, 2nd Earl of Douglas and by sources writing long after his death referred to from time to time as Earl of Mar in right of his mother; born c1358; knighted 1378; married c23 Sep 1371 Isabel, daughter of Robert II by his 2nd wife Eupheme, and dspsl, being killed at the Battle of Otterburn c19 Aug 1388, leaving illegitimate issue. [Burke's Peerage]
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From Doug Hickling [to someone else who had a diiferent Isabella as James' wife]
"You show that the wife of James (2nd Earl of Douglas) Douglas was Isabella (of Durisdeer) Stewart whereas both SP and CP say that she was Isabel Stewart, daughter of King Robert II by Eupheme of Ross, but the name of the spouse is irrelevant because James's sons and a daughter were all illegitimate! This is why he granted the lands of Drumlanrig to his bastard son William and why James, the second Earl, was succeeded by his cousin Archibald Douglas (the Grim), bastard son of James, Lord of Douglas, called the good Sir James who was in turn the son of Sir William (le Hardi) Douglas."
SP=THE SCOTS PEERAGE edited by Sir James Balfour Paul
CP=The Complete Peerage of England, Scotland, Ireland, Great Britain and the UK, Extant, Extinct or Dormant. By G.E.C, New edition, revised and much enlarged. Edited by The Hon. Vicary Gibbs, with the assistance of H Arthur Doubleday. 1916, St Catherine Press London