MLA

Catholic Church. Pope (1572-1585 : Gregory XIII, Gratian, 12th cent. Decretum Gratiani emendatvm, et notationibvs illvstratvm. Vna cum glossis, Gregorii XIII. pont.max. ivssv editvm. Ad exemplar romanvm diligenter recognitum.. 1588. https://collections.library.yale.edu/catalog/2041436.

APA, 6th edition

Catholic Church. Pope (1572-1585 : Gregory XIII, & Gratian, 12th cent. (1588)[Decretum Gratiani emendatvm, et notationibvs illvstratvm. Vna cum glossis, Gregorii XIII. pont.max. ivssv editvm. Ad exemplar romanvm diligenter recognitum.]. https://collections.library.yale.edu/catalog/2041436.