(Collecta: S. Marco)
The basilica seen from the back; it is built into old Roman buildings found within the Roman Forum (Image source)
(Image source
(Image source)
From the Churches of Rome wiki:
Santi Cosma e Damiano dates back to 527, when Theodoric the Great, king of the Ostrogoths, and his daughter Amalasuntha donated the library of the Forum of the Peace and a portion of the so-called Temple of Romulus to Pope Felix IV. The pope united the two buildings to create a basilica devoted to two Greek brothers and saints, Cosma and Damian, in contrast with the ancient pagan cult of the two brothers Castor and Pollux, who had been worshipped in the close by Temple of Castor and Pollux. The apsis was decorated with a Roman-Byzantine mosaic, representing a parousia (coming at the end of time) of Christ.
In 1632, Pope Urban VIII ordered the restoration of the basilica. The works, projected by Orazio Torriani and directed by Luigi Arrigucci, raised the floor level seven metres, bringing it equal with the Campo Vaccino, thus avoiding the infiltration of water. Also, a cloister was added. The old floor of the basilica is still visible in the inferior church, which is actually the lower part of the first church.
[...]
A masterpiece of 6th-7th century art is the mosaic. In the middle is Christ, with Saint Peter presenting Saint Cosmas and Saint Teodorus (right), and Saint Paul presenting Saint Damian and Pope Felix IV; the latter holds a model of the church.
