Jane Dymant alerted me (Rob Hoyt) to her obit: "Mary Belle (Mrs. Russell Adams) was in the Guardian January 13 1938, page 6."
"Prince Edward Island Baptism Card Index, 1721-1885," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/KCX8-4Q8 : accessed 16 Nov 2014), John Casely in entry for Mary Isabella Casely, 20 Mar 1869; citing p. 7, volume 1, Kensington , Prince Edward Island, Public Archives, Charlottetown; FHL microfilm 1487752.
"Canada Census, 1881," index, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MVN7-V5T : accessed 16 Nov 2014), Mary Caseley in household of John Caseley, Lot 18, Prince, Prince Edward Island, Canada; citing p. 47; Library and Archives Canada film number C-13162, Library and Archives Canada, Ottawa, Ontario; FHL microfilm 1375798.
It may be possible to confirm family relationships with Mary by comparing test results with other carriers of her mitochondrial DNA.
However, there are no known mtDNA test-takers in her direct maternal line.
It is likely that these autosomal DNA test-takers will share DNA with Mary: