In the south-west corner of the Piazzale delle Corporazioni a little shrine was inserted. It is particularly interesting because of the find of a marble altar in 1881. The original is now in the Museo Nazionale in Rome. A plaster-cast is in situ.

The Shrine of the Altar of the Twins 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. Against the back wall was a brick altar. 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 slab for offerings was fastened on top with lead. 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, found by shepherds. This story is situated near the Palatine, 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. He did not need these during his encounter with Venus.

The altar carries various inscriptions:

P(ublius) Aelius Trophimi Aug(usti) l(iberti) proc(uratoris) prov(inciae) | Cretae lib(ertus) Syneros et | Trophimius et Aelianus fili

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).

Dedicata k(alendis) octobr(is) | M(anio) Acilio Glabrione C(aio) Bellico Torquato co(n)s(ulibus)

The altar was dedicated on October 1, 124 AD.

Votum Silvano
[A]ram sac[6]onam Aug. genio[3]sacomar[---]

This inscription tells us that the altar was dedicated to Silvanus. It is not certain that the two lines belong together. In the second line there may be a mention of the annona (the Imperial grain supply of Rome), and of the Genius of the guild of the weight-makers and weigh-masters (collegium sacomariorum).

Decurionum decreto

The placing of the altar was approved by the city council, indicating that it was located on important, public ground.

The reliefs leave little doubt that the altar was dedicated to Mars. The first two inscriptions might be related. The dedication to Silvanus is usually seen as re-use of the altar, presumably the placing in the shrine on the Piazzale. We can only guess where the altar was originally located.

The representation of part 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 the sculptural group of Mars and Venus, found in the Aula del Gruppo di Marte e Venere (II,IX,3), which represents either the couple Marcus Aurelius - Faustina, or Commodus - Crispina.

In another Ostian inscription we hear of L. Calpurnius Chius, quinquennal(is) collegi Silvani Aug. maioris quod est Hilarionis iunctus (or functus) sacomari. Perhaps we should translate: "president of the larger guild of the August Silvanus, which is named after Hilario, charged with the weigh-duties". Again we find a tie between the weigh-masters and Silvanus, this time Silvanus Aug., probably a reference to the Imperial cult. Another Ostian inscription tells us about Cn. Sentius Felix, who is patron de sacomar.

It is somewhat surprising that the second dedication of the altar took place on the Piazzale. We know that the sacomarium or ponderarium was usually near the Market. Several Ostian inscriptions tell us that weights were in or near the Macellum (IV,V,2): pondera ad macellum. If there really is a mention of annona in the Silvanus-inscription, then we have a good explanation for the placing of the altar on the Piazzale, where references to grain are abundant. 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). See the Puteoli website.