The ivory has been cropped across the top; a fragment in the lower right corner has been lost and replaced.
The ivory shows two seated figures and an angel. On the left is St. Peter, recognizable by his curly hair and short beard; he sits in a highbacked chair with a dolphin-shaped armrest and raises his right hand in speech. On the right, an evangelist writes into his Gospel book. He has been identified as St. Mark because of the similarity of his features to those of Mark on the closely related ivories of the so-called Grado chair. The letter "A" in his book is the first letter of ἀρχή the opening word of Mark's Gospel. 
Between the figures is an elaborate inkstand; behind it stands the angel, who arches his wings above the seated saints and carries a staff; he turns toward Peter, presumably awarding him divine inspiration. The scene is framed by spiral columns, supporting an architrave inscribed ΠOΛIC RѠMH ("City of Rome"), evidently a reference to a lost cityscape above. (According to a tradition reaching at least as far back as Eusebius, St. Mark based his Gospel on the reminiscences and preachings of St. Peter in Rome.) This ivory is the earliest example of the rare type of evangelist portrait in which the evangelist is accompanied by an apostle. 
It has long been recognized that this plaque belongs to a group of fourteen ivories closely related to each other in style, size, and content, many of them illustrating events in the life of St. Mark. The date and the function of the ivories, however, have provoked considerable dispute. In the latest study, Weitzmann abandons both the early (sixth-century) and late (eleventh-century) dates hitherto proposed, and favors instead a seventh-century date, suggesting that the ivories are Syrian. He considers this ivory of Peter and Mark to be the latest of the fourteen ivories, because of the advanced mannerism of its style—especially the linear folds and the shallow space alloted to the figures. He has proposed that this plaque, along with the other ivories, once adorned a large wooden church door.