Pius X, P. (1904). The XIII Centenary of St. Gregory, Encyclical Letter of Pius X. New York.
Lua i Stíl Chicago (17ú heag.)Pius X, Pope. The XIII Centenary of St. Gregory, Encyclical Letter of Pius X. New York, 1904.
Lua MLA (8ú heag.)Pius X, Pope. The XIII Centenary of St. Gregory, Encyclical Letter of Pius X. New York, 1904.
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