Forum Boarium: Temple of Hercules Victor (or Olivarius) by ortygia on Flickr.

Via Flickr:
The Round Temple (Rundtempel) along the Tiber River in Rome, in the area of the Forum Boarium. The temple is a centrally-planned tholos type, one of the earliest structures in Rome to have been built of marble, in this case imported Pentelic marble from Greece. It is also one of the earliest Corinthian temples in Rome; the cella is surrounded by twenty columns. Probably dedicated to Hercules, with the epithet Victor or Olivarius. No blocks of the entablature survive. Another well-preserved temple from this period is the so-called Temple of Vesta at Tivoli.
The most important monograph on the temple is:
F. Rakob and W-D Heilmeyer, Der Rundtempel am Tiber in Rom (Mainz am Rhein 1973)