William B. Coster

William Bay Coster (1867 – December 19, 1918) and Marie Bay (née James) Coster (1841–1904), who were married in 1864. Among his siblings was brother Charles Coster, and sister Elizabeth Mary Coster, who married Alfred Egmont Schermerhorn. His father was a soldier and public official, who is best known for commanding a brigade at the Battle of Gettysburg.

His maternal grandfather was Augustus J. James of Albany, the brother of theologian Henry James Sr., His paternal grandparents were John H. Coster and Sarah Adeline (née Boardman) Coster, making his father a first cousin of fellow New York clubman, Harry Coster. His great-grandfather, John Gerard Coster, came from Haarlem in the Netherlands to the United States shortly after the Revolutionary War and founded the family fortune through the mercantile firm, "Henry A. & John G. Coster".

Career Coster became a stockbroker and opened up an office with his brother Charles and John M. Knapp in 1893. Although William was accused of wrongdoing, and, along with his other siblings, worked out of the same office as Charles, William himself was eventually declared by the New York Stock Exchange to be completely innocent in the debacle.

Coster also served in the New York National Guard as the aide de camp of the First Brigade, achieving the rank of captain in 1896.

Society life In 1892, Coster, one of the best-known bachelor clubmen, was included in Ward McAllister's "Four Hundred", purported to be an index of New York's best families, published in The New York Times. Conveniently, 400 was the number of people that could fit into Mrs. Astor's ballroom. Coster was a member of the Union Club. Coster was married to Maria "Minnie" Griswold Gray (1868–1947) at St. Saviour's Episcopal Church in Bar Harbor, Maine. Minnie, a close friend of etiquette author Emily Post, was the daughter of Henry Winthrop Gray and Mary (née Travers) Gray, and the granddaughter of William R. Travers. Her parents divorced and her father remarried to Matilda Frelinghuyhsen (daughter of U.S. Secretary of State F. T. Frelinghuysen ) in May 1889. Together, William and Maria first lived in New York, then Paris, and became the parents of three children:

Matilda Gray Coster (1901–1962), They divorced in 1928, and she remarried to Luis Martínez de las Rivas, in 1932. They later lived in Cuernavaca, Mexico. William Bay Coster Jr. (1908–1945), who rowed on the Pembroke College crew at Oxford and was an air-raid warden in London during World War II. His widow, who lived in Paris at 5 Rue Vaneau, died at her home, 39 East 79th Street in New York, at the age of 79 in July 1947.

Descendants Through his daughter Matilda, he was the grandfather of Mathilda Coster Mortimer (1925–1997), Duchess of Argyll. Mathilda was first married to Clemens Heller, founder of the Salzburg Global Seminar, a school in Salzburg, Austria.