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 known compitum-altars 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:

[---m]AG(ister/istri) D(e) S(ua) P(ecunia) F(aciendam) C(uravit/uraverunt) LARIBVS VICIN(alibus/is/iae) SACR(am) ARAM MARMOREAM

According to the inscription the marble altar was dedicated to the Lares by one or more magistri. The Lares are called Lares Vicinales, Vicinis, or Viciniae, "of the vicus, of the neighbourhood". All around the altar is a relief. The central scene is made up of a square altar, decorated with a garland and bucrania and with a fire on the top. To the right of the altar is Hercules. His head is missing. He stretches his empty right hand out towards the altar in an inviting gesture, from his left arm the lion-skin is hanging down. In his left hand must have been the club, but this is no longer recognizable. Behind him is a pig, with a ribbon around its body. Behind the altar is a tree, with a hairy object at the end of one of its branches. Against one of the other branches leans a thyrsus. There are two almost identical lateral scenes. On either side of Hercules Pan is seen. The Panes move towards Hercules and hold a situla. They are looking back at a dancing Lar. One hand of the Panes is behind the Lares and they are therefore apparently leading the latter towards the altar. The attributes of the Lares are lost. 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 Roberto 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).

3D-drawing of the compitum, from the south-east. The Antonine masonry is indicated in yellow. From Bakker 1994, fig. 17.

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, a small shrine 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 be an example of the pervia compita, "passable compita", mentioned by Persius and Calpurnius Siculus.

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