Mary was a Plantagenet Princess of England given that she was th in direct maleline descent from King Edward III of England Accordingly since the illegitimate children of John of Gaunt st Earl of Lancaster were later legitimized their maleline descendants should hold the title of PrincePrincess of England or very least of The House of Plantagenet Given that Mary SOmerset should be known as Princess of Somerset as the current Eugenie and Beatrice are Princesses of York and are not known of England