Marcellus was a Pope, so the statue at right properly gives him a triple tiara and a cross with three crossbars. But he has no individualizing attribute, and without a label or context of some sort he will be hard to identify. Sometimes he even wears a simple mitre rather than the papal tiara.
The historical Pope Marcellus served during the period following the persecution of Diocletian. Not only had the church of Rome lost many martrys and church buildings, but it faced a surge in conversions. Marcellus established a number of new churches and re-organized the clergy. Then an apostate from among the new converts denounced him to the emperor Maxentius (ruled 306-312), who sent the pope into exile in about the year 308 (Duchesne, II, cxix-c, 165-66.)
The 6th-century Liber Pontificalis suppresses the exile story in favor of one taken from a 5th-century Acta. In the new story the Emperor is Maximian, who ruled from 286 to 305. He has one of the new churches converted into a stable and condemns Marcellus to work there as a stable hand until his death (Duchesne, ibid., 164).
The Golden Legend has a brief notice on St. Marcellus that repeats the stable story and dates the saint's death as 287.
Prepared in 2014 by Richard Stracke, Emeritus Professor of English, Augusta University