(Collecta: S. Marco)
(Image source)
From the Churches of Rome wiki:
Legend claims that Pope St Marcellus (308-309) was sentenced by Emperor Maxentius to look after the horses at the station of the Imperial mail on the Via Lata, where the Via del Corso now lies. He was freed by the people, and hidden in the house of the Roman lady Lucina (see also San Lorenzo in Lucina). He was rearrested, and imprisoned in the stables.
A church was built at or near the site of the stables in the 4th century, and Pope Boniface I was consecrated pope here in 418. It was one of the tituli, the first parish churches of Rome, known as the Titulus Marcelli. The church was restored by Adrian I in the 8th century. This structure lies below the present church, and has been excavated. The excavations are still not open to the public.
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Beneath the high altar is a marble memorial stone from the 3rd century. The front is decorated with 12th century opus sectile, and it contains relics of martyrs, including Pope St Marcellus.
