Renaissance art historians conventionally work in terms of types. Artistic production to a large extent can be thought of in terms of basic forms or categories—portrait, altarpiece, devotional image, etc.—customized according to the requirements of patrons. The artistic culture of Venice in the late fifteenth and early sixteenth century saw the production of many objects that frustrate that approach by being insistently sui generis. Among them are a pair of marble reliefs: one signed by...