The Complete Peeragedownload available here(click "here", when it opens, then click download, symbol 6 from the left on new window taskbar, pdf.) which says here succession to the Earldoms of Caithness and Orkney see ante, vol. ii, sub CAITHNESS, pg 476; vol. x, Appendix A, p. 31; Scots Peerage, vol. ii, p. 320. particularly in Complete Peerage, Vol X, Appendix A, pg 30. says "last of the Angus line", and was succeeded by Malise, 8th Earl of Strathearne.
Sources
Ferteth 2nd Earl of Strathearn, "Scots Peerage", Balfour-Paul; Founded on Wood's edition of Sir Robert Douglas's Peerage of Scotland; Containing an Historical and Genealogical Account of the Nobility of That Kingdom by Sir James Balfour Paul (1846-1931); free download, Borrow, & Streaming : internet archive accessed 16 Mar 2020 v8 p240
The Scots Peerage : Founded On Wood's Ed. Of Sir Robert Douglas's Peerage Of Scotland; Containing An Historical And Genealogical Account Of The Nobility Of That Kingdom : Paul, James Balfour, Sir, 1846-1931 : Free Download, Borrow, And Streaming : Internet Archive". Internet Archive. Accessed March 16 2020.>Ferteth 2nd Earl of Strathearn, Scots Peerage, Balfour-Paul, Vol 8, pg 240
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Maurice Moray of Drumsagard married Joanna Menteith widow of Malise of Strathern the father of the above Malise 8th of Strathearn. He did not marry Isabella. Maurice was stepson of Mary Strathearn who was step-daughter to Joanna Menteith!
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