Meanwhile he [Desiderius] sent envoys to Constantinople to hire artists who were experts in the art of laying mosaics and pavements.  The [mosaicists] were to decorate the apse, the arch, and the vestibule of the main basilica; the others, to lay the pavement of the whole church with various kinds of stones.  The degree of perfection which was attained in these arts by the masters whom Desiderius had hired can be seen in their works.  One would believe that the figures in the mosaics were alive and that in the marble of the pavement flowers of every colour bloomed in wonderful variety.  And since magistra Latinitas had left uncultivated the practice of these arts for more than five hundred years and, through the efforts of this man, with the inspiration and help of God, promised to regain it in our time, the abbot in his wisdom decided that a great number of young men in the monastery should be thoroughly initiated in these arts in order that their knowledge might not again be lost in Italy.

[…] the apse and the major arch he faced with mosaic.  He ordered the following verses to be written in large letters on the arch:

In order that under Thy Leadership the just may be able to reach and take possession of the heavenly home,

Father Desiderius founded here this hall for Thee. [Holt notes that these verses copy the inscription which Constantine had set on the arcus maior of the Vatican basilica].

In the apse, under the feet of St. John the Baptist and St. John the Apostle, he ordered the following verses to be written:

This house is like Mount Sinai which brought forth sacred laws.

As the Law demonstrates what was once promulgated here.

The Law went out from here which leads the minds from the depths and having become known everywhere, it gave light through the times of the age. [Holt notes that first two verses in imitation of those on the walls of the Lateran basilica].

[…]  He further decorated the arches above the entrance and the vestibule of the church with beautiful mosaics. […]  Also he had the outside of the arches covered with mosaic and the verses of the poet Marcus inscribed there in golden letters. [verses can be found in Migne, pat. Latina, 80, p. 183].