A section of the space behind the octagon. I was afraid I had overlooked the tetraconch, but to my relief found back home that this must be it. The Wikipedia explains: "A tetraconch, from the Greek for "four shells", is a building, usually a church or other religious building, with four apses, one in each direction, usually of equal size. The basic ground plan of the building is therefore a Greek cross. They are most common in Byzantine architecture, and related schools such as Armenian and - it has been argued that they were developed in these areas or Syria, and the issue is a matter of contention between the two nations in the Caucasus." On the plan it has number VII.