MLA

Catholic Church. Pope (1572-1585 : Gregory XIII, Gratian, 12th cent. Liber sextvs Decretalivm d.Bonifacii papae VIII. Clementis papae V. Constitvtiones. Extravagantes tvm viginti d.Ioannis papae XXII. tvm commvnes.. 1588. https://collections.library.yale.edu/catalog/2041437.

APA, 6th edition

Catholic Church. Pope (1572-1585 : Gregory XIII, & Gratian, 12th cent. (1588)[Liber sextvs Decretalivm d.Bonifacii papae VIII. Clementis papae V. Constitvtiones. Extravagantes tvm viginti d.Ioannis papae XXII. tvm commvnes.]. https://collections.library.yale.edu/catalog/2041437.