Mary Brandon, Lady Monteagle, daughter of Charles Brandon, step-daughter of Mary Tudor by Hans Holbein the Younger
Her Wikipedia article is here.
From lisby1's photostream on flickr.
Lady Monteagle wears a dress with a square neckline and a straight coiffure under a French hood. While her dress looks plain, there are hints of beading or pearls on her headdress and neckline and the medallion is suspended from a barely-visible necklace. She wears a beaded "M" suspended from her upper necklace.
Keywords: Holbein, Brandon family, Stanley family, Baroness, English, Lady-in-Waiting, Jane Seymour, straight coiffure, French hood, jeweled headdress, veil, square neckline, full sleeves, necklace, earrings, jeweled neckline