Another text in the PG 157 volume (col.725) is one describing the tombs of the emperors in the Church of the Holy Apostles, here:
On the tombs of the emperors which are in the Church of the Holy Apostles.
In the Heroon of the great and holy Constantine towards the east end lies the porphyry larnax of the great Constantine himself, in which he himself is deposited with his mother Helena.
Another porphyry larnax, in which is deposited Constantine, son of the great Constantine.
Another porphyry larnax, in which is deposited Theodosius the Great.
Another porphyry larnax, in which is entombed Theodosius Junior.
Another larnax of green Thessalica, in which lies Zeno. …
And so it continues, including Heraclius, and various Byzantine rulers. On col. 735 we read, after a stoa – literally a portico, but by this period any building or chamber containing columns – with three small larnaxes containing the remains of Arcadius and his wife and son:
Another stoa in the same church towards the west end, in which lies an larnax of expensive Roman stone, where is deposited the accursed corpse of Julian the Apostate.
UPDATE (18/12/2013): I have updated this to reflect the meanings of the words “stoa” and “larnax”, after reading more around the subject. While the PG renders larnax as “urn”, it should be rendered “sarcophagus”. “Roman stone” means porphyry. “Thessalica” is thessalian marble.