The Barnum Family, 1517-1904 calls him Allan Romaine Barnum, with the note "Spells his name thus." The Barnum Family, 1350-1907 calls him Allan Romine Barnum, but later provides an errata note that the name should be Allan Romayne Barnum.
Allan Romine Barnum died April 1, 1961 in Norwalk, Fairfield County, Connecticut. This is the spelling of his name as it appears on his World War I Draft Card--viewable on Ancestry.com.
From The New York Times of April 3, 1961: Norwalk, Conn, April 2--Dr. Allan R. Barnum, a dentist for nearly sixty years, died here yesterday at his home, 15 Marian Avenue. His age was 88. He was a cousin of PT Barnum [N.B., 1st cousin, twice removed].
Allan Romine Barnum died 4/1/1961 Norwalk, Fairfield County, Connecticut (Connecticut Death Index).