Church of Santa Maria sopra Minerva
The legend in the upper right corner identifies SAN BARNABAS AP[OSTO]LVS, St. Barnabas the Apostle. The book he is holding is open to the words MAGISTRA APOSTOLORVM (see detail). Magistra is the feminine form of magister, "teacher" or "master''; apostolorum means "of the apostles." Thus the phrase means "(female) teacher of the apostles" or "mistress of the apostles." The phrase is sometimes used as a title of either the Virgin Mary or the Church.
But could the female figure in this painting in fact represent the Virgin Mary? The latter does usually wear a blue mantle like the one in the painting, but she is never shown bare-headed unless (a) the subject is the Annunciation or some earlier event in Mary's life, or (b) her head is about to be crowned.
Usually an uncovered head of flowing hair identifies St. Mary Magdalene, but I have been unable to find any text that associates her with either St. Barnabas or the phrase magistra apostolorum. She is called an apostle in The Life of St. Godric, where she appears to St. Godric in a dream with the Virgin Mary, who calls her "a woman apostle among the apostles" (Stouck, p. 423).
More of St. Barnabas
More of St. Mary Magdalene
More of the Virgin Mary
More of the Church
Photographed at the site by Richard Stracke