In Pompeii the Macellum was a large building occupying the north-eastern comer of the Forum. Excavated between 1818 and 1822 it was first thought to be a temple (Temple of Augustus or Pantheon). Only later was its real function acknowledged. A nearby building, the Granary of the Forum (VII 7, 29), was probably the market for grain and legumes (forum olitorium). The Macellum of Pompeii originally dates back to the late Samnite period, that is the second half of Il century b.C. Subsequently it was rebuilt in the same place and with the same layout in the period of the Julio-Claudian dynasty. At the moment of the eruption in AD 79 reconstruction of the building was in progress and it was probably not all in working order. The building consists of a wide porticoed courtyard with three gates.

The main gate had ttivo doors and opens onto the western side where the shops of the Forum were located. The second gate opens onto the northern side in between a series of shops bordering Via degli Augustali. The third one opens up onto the south-eastern corner on a side alley. The courtyard was decorated by fresco paintings of the Fourth Pompeian stylc on the northern and western side, with panels containing pictures or individual subjects, alternating with splendid works of architecture. On the top there were still Iives with fish, birds and pottery, objects relating to the function of the building. The surviving mythological pictures depict lo and Argus on the western side and Ulysses and Penelope on the northern side while those representing Medea and her children, Achilles and Tethys, and flying Frissus have been lost.
In the Macellum the 11 shops along the southern wall of the portico and the dodecagonal pavilion (tholos) in the centre were devoted to sale. Shops had an upper floor provided with a gallery.

The tholos (a circular building in wood with a cone cover supported by poles and with a pebble floor) was being refurbished. New foundations and a new floor had already been constructed and 12 stone basements had already been laid in the corners to support a colonnade.
On the eastern wall there are three rooms whose function has long been debated. The room in the middle is a chapel dedicated to the worship of the Emperor as demonstrated by an arm of a marble statue holding a globe in its hand. In the niches in the walls of the cella two marble statues were found (now in the Archaeological Museum of Neaples), representing a man and a woman, interpreted as members of the Imperial family or of a Pompeian family who had payed the works of refurbishment of the Macellum.

The room to the north-cast also had a religious function as it housed a small temple and a low altar. More doubtful is the function of the room located in the south-eastern corner. At first it was interpreted as a banquet hall for religious ceremonies. Today it is considerecf a place for selling fish. The walls were covered with paintings which havc been lost and a picture depicting the river Sarno and other local deities.
In a small enclosure in the north-eastern corner of the portico the skeleton of a goat was found, destined for sacrifìce rather than sale. In the water drains of the tholos fìshbones and scales were found, probably dating to the preceding phases of the pavilion. The shops outside the Macellum, however, were operational and numerous vases containing foodstuffs were stocked in them. There was also one Macellum in Herculaneum as indicated by some inscriptions discovered in the excavations carried out in the 18° century.

Source
Soprintendenza Archeologica di Pompei