The Gridiron Apse in the Mausoleum of Galla Placidia

5th century
Mausoleum of Galla Placidia

At the end of the apse is the mosaic of the deacon. The deacon stands at the right holding a processional cross and a book. On the left is an open bookcase with the four Christian gospels. In the center is a gridiron like those on which the deacons Lawrence and Vincent are said to have been martyred. (See this page for a close-up and discussion of which deacon is being represented.)

Above the apse two apostles flank a small fountain with two birds.

Other photographs:

More of St. Lawrence
More of St. Vincent of Saragossa

Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.

1 Mackie has suggested that the saint is not Lawrence but Vincent of Saragossa.