In the south-west corner of the Piazzale delle Corporazioni a little shrine was inserted, the Shrine of the Altar of the Twins. It is particularly interesting and still enigmatic because of the find of a marble altar in 1880 or 1881 by Lanciani. The original is now in the Museo Nazionale in Rome. A plaster-cast is in situ.
The shrine is a little rectangular room (4.45 x 3.48 m.). The masonry has not been dated. We only know that it is later than Hadrian (117-138 AD). There is a similar room on the east side of the square. Set against the back wall is a brick base (1.70 x 1.35, h. 0.85). Along the walls were benches with marble revetment.
The room was ransacked at an unknown point in time by people who entered through a breach in the west wall. They missed the altar, which was covered by a large piece of the ceiling. The altar measures 0.84 x 0.84 m., and is 1.10 m. high. A container for offerings or a statue was fastened on top with lead (the altar is explicitly called "altar" in an inscription, but it may have been re-used as a statue-base). The four corners are decorated with ram's heads and wreaths. The front has a depiction of Mars, Venus (with goose and Amor), and Hymenaeus (the god of marriage). On the back we see Romulus and Remus, suckled by the she-wolf. They are found by two shepherds, Faustulus and Faustinus. This story is situated near the Palatine (the Lupercal), and the personification of the hill can be seen in the upper left part. In the lower right part is the personification of the Tiber. Jupiter's eagle is present as well. On the sides of the altar are amorini, hauling the weapons and chariot of Mars. The amorini are moving towards the front side of the altar.
The altar carries various inscriptions. On the front side, above Mars and Venus:
The people responsible for the altar were P. Aelius Syneros (freedman of P. Aelius Trophimus, himself freedman of Hadrian and procurator of Crete) and his sons Trophimius and Aelianus (named after the former dominus of Syneros).
On the lower part of the front side:
The placing of the altar was approved by the city council, indicating that it was located on important, public ground.
On the upper part of the front side, on a small ledge:
Perhaps the annona (the Imperial grain supply of Rome) and the Genius of the guild of the weight-makers and weigh-masters (collegium sacomariorum) were mentioned. The missing six letters are perhaps to be restored as ad Annonam, and might be a reference to the temple (of Annona?) in the centre of the Piazzale delle Corporazioni.
On the upper part of one of the sides:
This inscription tells us that the altar was dedicated to Silvanus.
On the lower part of the other side:
The altar was dedicated on October 1, 124 AD.
The reliefs leave little doubt that the altar was originally dedicated to Mars. If one of the inscriptions really contained the words ad annonam, then the altar may originally have been located near the temple in the centre of the Square of the Corporations, in a setting so that all sides could be seen (which was not the case in the little shrine). The representation of the founding of Rome, under the supervision of Jupiter and with reference to the concordia of Mars and Venus, suggests a link with the Imperial cult. We are reminded of a sculptural group of Mars and Venus that was found in the Aula del Gruppo di Marte e Venere (II,IX,3), representing either the couple Marcus Aurelius - Faustina, or Commodus - Crispina. The statue seems to have been dumped in the Aula; had it been standing on the base in the back of the shrine, behind the altar?
The dedication to Silvanus is usually seen as re-use of the altar by the sacomarii. In another Ostian inscription we hear of L. Calpurnius Chius, quinquennal(is) collegi Silvani Aug. maioris quod est Hilarionis iunctus sacomari. Perhaps we should translate: "L. Calpurnius Chius ... president of the larger guild of the August Silvanus, which is named after Hilario, member of the guild of the weigh-masters". Again we find a tie between the weigh-masters and Silvanus, this time Silvanus Aug., a reference to the Imperial cult . We may note that L. Calpurnius Chius was also president of the guild of the grain measurers (mensores frumentarii). In Ostia's sister-city Puteoli we hear of people who call themselves mensor et sacomarius, and in the same city the sacomarium seems to have been in the harbour district (emporium). Another Ostian inscription tells us that Cn. Sentius Felix was patron de sacomar.
The altar was found with the back side turned towards the front, witness an illustration by Lanciani, the name given to the shrine by the excavators, and the orientation of the plaster cast. The present author would like to relate this to the state of preservation of the altar. The back side is preserved quite well, the other sides are damaged severely, and this can hardly be the result of the collapse of the roof of the tiny shrine. The damage, particularly of heads, suggests that the altar, with its references to the Imperial cult, was attacked by Christians in the fourth century AD. The back side, depicting the foundation of Rome, may have been spared. The Ostian magistrates may have restored the altar with the more neutral side as front side. We may think here of Ragonius Vincentius Celsus, prefect of the grain-supply, who erected a statue of Roma in front of the theatre. Perhaps now, in the late fourth century, the altar became the base of a statue of Roma.
Plan of the shrine.
After SO I.
 See the description of the theatre (note 1) for a possible relation between the altar and the theatre: refounding of Rome; (guild of) Silvanus.
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