Edward Adolphus Ferdinand Seymour, Earl St.Maur (17 July 1835 - 30 September 1869) was the eldest son of Edward Adolphus Seymour, 12th Duke of Somerset. He was known as Lord Seymour until 1863 when his father was created Earl St. Maur of Berry Pomeroy and he adopted his father's new creation as a courtesy title. He was commonly known as Ferdy.

Ferdy briefly fought as a volunteer in the Persian War (1855-57) and almost immediately afterwards, was at the Relief of Lucknow during the Indian Mutiny (1857-1858). He fought in Italy and Sicily, as a civilian volunteer, joining Dictator General Giuseppe Garibaldi's Essercito Meridionale (Southern Army) as a private in 1860. At an early stage he assumed the rank of Captain on the basis that along with his younger brother Edward Percy St. Maur (1841-1865) he had co-commanded Volunteer Cavalry in England; both brothers assuming the rank of Captain. He distinguished himself in the Volturno campaign in late 1860 and in other campaigns. Garibaldi later officially conferred him with the rank of Captain despite the fact that Ferdy was merely a civilian volunteer, and he assumed the name pseudonym 'Captain Richard Sarsfield' in commemoration of a hero he had heard about when at Christ Church College, Oxford.

In late 1860, after Garibaldi had appointed Ferdy as his 'Military Secretary', he accused a brother officer (who happened to be a favourite of Garibaldi's) of embezzling Garibaldi funds, the said brother officer challenged Ferdy to a duel that his superior officer (Colonel John Whitehead Peard - Garibaldi's "Englishman") forbade him to attend. Ferdy felt it necessary to be accompanied at all times by bodyguards and to escape back to Britain - which he did. The scandal induced Ferdy to give up any thought of involvement in warfare and he was never again to partake in warfare, choosing to travel extensively throughout Europe and study languages, seldom returning home, much to the resentment of his family.

In 1866 he began a relationship with a 17-year-old maid called Rosina Swan. The Earl took Rosina with him during his travels, returning to England with her in 1868 to live near Brighton. Ferdy and Rosina had two children; a girl Ruth (1867-1953) was born whilst the couple were in Tangier and a boy Richard Harold St. Maur (1869-1927) born in Brighton. A few months after the birth of his son Ferdy died during a botched emergency tracheotomy at his own flat in Mayfair. Earl St. Maur died shortly after the birth of his son. If Ferdy had married Rosina, Harold would have now been the heir to his grandfather's dukedom and he spent many years trying to prove that a marriage had taken place. Looking for a possible Dutch witness to the marriage by the name of Ravesteyn, he even published an advertisement in a newspaper in the Netherlands in 1924, offering a reward of £50 for proof of the fact without success. In 1885 the 12th Duke died. He had outlived both of his sons (Ferdy's brother, Edward having died in 1869). The 12th Duke's brother (Archibald Archibald Henry Algernon Seymour ) became the 13th Duke of Somerset.