Willement, Thomas. Regal heraldry; the armorial insignia of the Kings and Queens of England, from coeval authorities, London: W. Wilson; Rodwell and Martin. 1821. pg 14, 25. Regal Heraldry
books.google.com
"Even her own uncle Lancaster came to regard her as an enemy." Isabella's mother, Joan of Navarre, was Thomas of Lancaster's older half-sister. [1]