Her maiden name was Lady Mary Stanley, she married Thomas Egerton, 2nd Earl of Wilton. Her Peerage.com article follows, "Lady Mary Margaret Stanley was born in 1801.1 She was the daughter of Edward Smith-Stanley, 12th Earl of Derby and Eliza Farren.2 She married Thomas Grosvenor Egerton, 2nd Earl of Wilton of Wilton Castle, son of Robert Grosvenor, 1st Marquess of Westminster and Lady Eleanor Egerton, on 29 November 1821. She died on 16 December 1858.
Her married name became Egerton."
Upgrade image posted 3 January 2012 from Christie's; spots and flaws, mainly in the background, removed with Photoshop by gogm.