Ok now, turn your back on the Temple of Saturn and walk back over to the right side of the Rostra. Below the railing you will see the remains of a small room with remains of a marble paved floor and some upright marble wall slabs. That is the 'SCHOLA XANTHA', 'Schola' means basically in this case a business office for the clerks and bean counters in charge of 'public works'. And 'Xantha' is a modern name given to this building based on an inscription (a person's name), they don't know what it was actually called in ancient times.
Now remember originally at this end of the Rostra ||||||| Emperor Augustus setup the Milliarium Aureum o in 20 BC to match the Umbilicus Urbis o at the other end of the Rostra, like this o|||||||||o. But it seems that during the reign of Emperor Tiberius (Augustus' successor) it was moved across the street and a small building was built here right against the Rostra, like this o|||||||||==. Ok now, why put a business office in this prime location? Because these boys are the clerks, copyists and herolds of the 'Curule Aediles' in charge of public works like aqueducts, sewers, markets, temples, public games, festivals and many etc's possibly even brothels :-) . So you got money going out and in on these government contracts and the money is kept across the street under the Temple of Saturn in the treasury. So it's an excellent location to transact this exchange, I mean ya don't want gold, silver and coins *accidently* falling off the cart in transit :-) . Plus it puts these government contract money exchanges in public view with the appearance of honestly, the shady dealings have already been made behind closed doors anyway :-) .
This building was first excavated in 1540 when they were looting the Forum of its marble for Renaissance buildings and lime kilns. So much more once existed but was destroyed and its location was even lost. And later for a long time it was believed that the seven chambers beneath the 'Porticus Deorum Consentium' was actually the Schola Xantha. But in that 1540 excavation a double inscription on the architrave over the entrance was found. It was destroyed but they did record the inscription. It said that during the reign of Emperor Tiberius (14-37 AD) an Imperial Freedman named Bebryx along with 'Aulus Fabius XANTHUS' (where they got the modern name XANTHA) built this Schola for the clerks and herolds of the Curule Aediles. This was a very elaborate office the inscription goes on to say that "placed there were seven silver images on brackets, a bronze table, faced the walls with marble, erected a statue of Victory and furnished it with bronze seats and several ornaments". The second later inscription was added during the reign of Emperor Caracalla (211-217 AD) and said that 'C. Avillius Licinius Trosius' restored the schola.
Also if you look at the right end of this building you will see a bricked half circle (think small brick pizza oven) which is below its floor level. That is a hypocaust which is actually a wood-fired furnace. The building's floor is raised and supported on small brick piers which create a large void below the floor. A fire is built just inside this half circle opening which heated the floor in cold or damp weather. Often clay pipes were built within the walls to heat them also as the heat and smoke vented out. There was most likely a floor/structure above this section and slaves would be in this basement tending this furnace. So perhaps this section was another small room of the schola?
Here's a photo of the Schola remains (it's the room behind the brick wall). We're facing north in this view, and the Temple of Saturn is on the left (not shown). The space in front of the wall is right at the corner of the Forum. That corner is empty except for some fragments and it's below ground level (hole). Ok so now we are standing in the SW corner of the Forum Square. The Via Sacra comes up from the other end of the Forum Square and runs between the Forum Square and the Basilica Julia, then it hits the side of the Temple of Saturn. At this point the street 'T's, if you go LEFT that is the Vicus Jugarius which runs between the end of the Basilica Julia and the side of the Temple of Saturn and out of the Forum. If you go RIGHT that is the beginning of the Clivus Capitolinus which goes around the Temple of Saturn and up to the Capitoline Hill. The Triumphs (military victory parades) would come up the Via Sacra and turn right on the Clivus Capitolinus and up to the Temple of Jupiter on the Capitoline Hill.
Next: #14.2: Clivus Capitolinus Viaduct