Lorenzo Veneziano, Annunciation Polyptych: Detail, St. Macarius
The inscription identifies this saint as "Macarius." There are several saints by that name, but the tau-topped staff and the tau cross on his habit identify him specifically as Macarius the Great, who led a monastery in the Egyptian desert in the 4th century. The tau's refer to the assertion by some early writers that Macarius was a disciple of St. Anthony Abbot, who is also often identified by a tau cross on his habit and/or his staff.1
In a few icons of this saint that I have examined he holds an open scroll. This is the one case I have seen where the scroll is closed. It most likely refers to the numerous analects attributed to him.
The feast of St. Macarius is celebrated on January 15.
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Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.