Mary was a Plantagenet Princess of England given that she was 6th in direct male line descent from King Edward III of England Accordingly since the illegitimate children of John of Gaunt 1st Earl of Lancaster were later legitimized their male line descendants should hold the title of Prince Princess 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