They put a trench into the forecourt in 1935 and came up against stone that had nothing to do with the building standing over it. Go round to the west end, to the sunken patch below pavement level, and look down: broken blocks, column bases, a flight of steps that goes down to nothing now. Along one block a frieze of lambs, twelve of them, walking in a row, fleece cut in little hooked curls, the spacing kept even while the legs are let to vary. Someone decided twelve and then had to carve twelve.
That's the church inaugurated in 415, Theodosius II's rebuild after fire took the first one. It lasted a bit over a century and burned as well, in 532, and what came after is the thing everybody walks in to see. The lambs sit out in the weather at ankle height, and most people step round them on the way to the door.
Twelve lambs is almost certainly the apostles, Fabien — that was the usual shorthand before anyone worried about it, with Christ as the thirteenth lamb or the shepherd. A council in Constantinople in 692 asked that Christ be shown as a man rather than a lamb from then on, though the carvers out in the weather kept their flock either way.
Three churches, one name, and the middle one's down to about a metre of stonework you can stub your toe on. Fabien, those lambs have outlasted two fires and the sightline of nearly everyone who walks past them.