Trinity Altarpiece: Detail, the left side
On the top row are St. John the Baptist (far right, with the lamb and book) and prophets and patriarchs from the Old Testament. Abraham stands behind John, and behind him is Isaiah. The others include Elijah, Daniel, Ezra, and two others whose inscriptions I can't make out.
Apostles and evangelists occupy the next row down. The inscriptions are garbled in several cases but comparing them with the attributes and the figures on the right side of the altarpiece we can identify (left to right) Mark, Thomas(?), Matthias (axe), Matthew (halberd), Andrew (cross saltire), James the Greater (scallop shell on hat), John the Evangelist (beardless, but why the palm branch?), and Peter (keys). In the corresponding row on the left side, St. Philip is erroneously labeled ANDREAS, "Andrew." But clearly the apostle on this side with the cross saltire is Andrew; that kind of cross is not seen with any other apostle.
On the next row down are the martyrs, their inscriptions also garbled in some cases: Honoratus (bishop's garb), Edmund (English king), George (red cross, dragon), Narcissus (bishop's garb, iron comb), Clement (papal crown), Vincent (palm branch, dalmatic), Marcellus (papal crown), Stephen (rock on bloody forehead). The inclusion of a bishop named "Narcissus" among these martyrs is probably due to a confusion between the St. Narcissus who was Bishop of Jerusalem in the 3rd century and died a natural death, and the St. Narcissus who was axed to death in the 4th century for refusing military service that would entail worshiping idols. His brother Marcellinus was beaten to death with canes (virgis crudeliter caesus), which perhaps explains the iron comb in this image. His story is in Acta Sanctorum, Jan. vol. 1, 83.
After the martyrs come the confessors (saints who were not martyred): Onuphrius (hair all over body), Francis (brown Franciscan habit, wound on hand), Benedict (black Benedictine habit, crozier, tonsure), Ambrose (cope, mitre, crozier), Jerome (red garb), Anthony Abbot (tau-top staff), Louis of Toulouse (beardless youth in pontificals), and Martin (two fingers held up). A few portraits of St. Martin through the ages have him noticeably pointing up with one or sometimes two fingers. The gesture may be a reference to his notable resistance to Arian teachings regarding the Son of God. (See Sulpitius Severus, VI.)
Finally, the bottom register is for female saints: four virgin martyrs with palm branches (one with an obscure inscription, then Dorothy, Agatha, and Margaret with her cross), Elizabeth of Hungary (Franciscan habit with triple-knotted cord, crown), Helena (crown, wooden cross), the virgin martyr Catherine of Alexandria (palm branch, crown, wheel in left arm), and Anne (in her arms the child Mary learning to read).
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Photographed at the Metropolitan Museum by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.