Clara Edith Graves Baker, 78, the librarian emeritus of the Society of Mayflower Descendents of the District of Columbia and a genealogist, died of cardiac arrest Thursday at Sibley Memorial Hospital.

Mrs. Baker, who was born in Washington and lived here all her life, became librarian of the society about 1970, and was named librarian emeritus in 1978.

She became interested in genealogy because she was decended from nine singers of the Mayflower Compact. She wrote a two-volume work, "Genealogy of Herbert Cornelius Graves," who was her father, and had it published privately. In addition to her own work, she assisted many others in genealogical research.

She was a member of the Mayflower Society, the national Huguenot Society and its Washington chapter, the Society of the Descendents of the Colonial Clergy; the Flagon and Trencher, an organization of the descendents of inn-keepers in colonial times, and the Stetson Kindred of America, a family organization. She was listed in the Hereditary Register of the United States of America, a registry of genealogists.

Mrs. Baker was a graduate of Eastern High School here. She graduated from Goucher College in 1924 and in the same year went to work in the Hygienic Laboratory of the Public Health Service. She also earned a master's degree in zoology at American University. In 1931, she resigned from the government.

Survivors include her husband, Arthur A. Baker, whom she married on Sept. 29, 1925, of Washington; a daughter, Carolyn B. Cowherd of Germantown, Tenn.; two sisters, Dr. Lucy Graves Taliaferro of Clearwater, Fla., and Asenath Graves McKnight of Washington, and four grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Washington Cathedral, or to the Heart Fund.