I have no proof of the following and make no claim here but am copying from Royal Descents: Scottish Records, 1.How to trace a descent from royalty> by The Reverend W.G.D. Fletcher, M.A., F.S.A.
1908 Chas A. Bernau, Waltor-onThames, England. Dunn, Collin & co. Printers, St. Mary AXE, London, E.C.
(This book was seen at, I believe, the LDS Library years ago.You must do your own research on the authenticity of the following information.This is all I have.)
Unbroken Male Descents from Royal Personages:pp 18,19 ". . .It is from one or other of these twelve royal princes and princesses that royal descents must be traced.There are, I think, only three families living today, whose male ancestors directly married a princess of the royal blood of England; and these are the families of Nevill, Marquis of Abergavenny, St. Lawrence, Earl of Howth, and Beaumont, Baronet.Ralph Nevill, 1st Earl of Westmorland, married Joan Beaufort, daughter of John of Gaunt; Sir Robert St. Lawrence, 15th Baron of Howth, Married Joan Beaufort, second daughter of Edmund Beaufort, Duke of Somerset, grandson of John of Gaunt; and John Beaumont, Lord Beaumont, married Eleanor, daughter of Henry Plantagenet, Earl of Lancaster, grandson of HenryIII.All other historic houses that intermarried diectly with royalty have become extinct, and it is now only through a succession of female lines that a descent from royalty can be traced."