Ostia's best preserved compitum (a shrine at crossroads) is on Piazza dei Lari, a small square to the south of the House of Diana (I,III,3-4). The oldest evidence on this small square is a late-Augustan or Claudian marble altar with a relief and an inscription. It should be noted, as Floriani Squarciapino has remarked, that it is round: all other compitum-altars which have been found are square. What the Piazza dei Lari looked like when the altar was placed there is not known. All the surrounding buildings are Hadrianic or Antonine and the older strata have not yet been investigated. It may be of importance that the altar is found at the intersection of two roads, Via di Diana and Via dei Lari, which coincide with the beginning of the inner pomerium of the Castrum. On the altar is an inscription:
According to the inscription the altar was dedicated to the Lares Vicin. by one or more magistri. The Lares are Lares Vicinales, Vicinis, or Viciniae. The central god in the relief on the altar is Hercules, standing next to an altar. A thyrsus close to Hercules refers to Liber Pater. On either side of Hercules Pan is seen, leading a Lar Vicin. towards him. The altar was at some point in time mutilated, presumably by Christians.
To the south of the altar is a rectangular basin. To the north is a small, rectangular edifice, probably from the Antonine period. To this phase belong four corner piers and two short stretches of wall in between the corner piers, in the long sides. Similar corner piers, but a little thinner, belonging to the first floor, are reported by Paribeni. It is not known whether the building had a roof. No staircase was found. The foot used was c. 0.303 long. The thickness of the walls is one and a half feet. The corner piers are two times three feet long on the outside, two times one and a half feet on the inside. The short stretches of wall in between the corner piers are four feet long. The entrances in the short sides are fifteen feet wide, the four lateral ones nine. The whole edifice is a rectangle of 28 x 21 feet (4:3).
There are at least two further alterations to the building (c. 250-275 AD?). Several entrances were narrowed or closed off, and the room was apparently divided in a northern and southern half. A small room was set against the outer south wall, between the building and the altar. Three L-shaped piers were set against the outer north and east wall, a further pier was erected near the northeast corner. Paribeni suggests that the piers supported a balcony.
The room is called "una specie di chiosco" by Paribeni, which is not very helpful. At first sight the wide entrances suggest that the edifice was a shop. This is unlikely however. The walls on the ground floor were almost entirely made up of entrances. There was therefore no storage space and the room must have been very draughty. This is why I think that a shop-threshold, placed in a door in the east wall, must belong to one of the later building phases, when several entrances were blocked. The curious lay-out of the original edifice and the close proximity to the altar justify the hypothesis that it was a sacellum, related to the altar, and probably replacing a shrine from the first century AD. The many wide entrances are probably to be understood as ritual gates. The structure seems to belong to the class of the pervia compita, mentioned by Persius and Calpurnius Siculus.
3D-drawing of the compitum, from the south-east.
From Bakker 1994, fig. 17.