In classical antiquity, the mouth of the loculus might be closed with a slab, plain, as in the Catacombs of Rome, or sculptural, as in the family tombs of ancient Palmyra.
The passage describes how the entrance to a loculus (a burial chamber) was often sealed in classical antiquity. It can be closed with either a plain slab, as seen in the Roman Catacombs, or a sculpted design, like those found on tombs of ancient Palmyra's families.