This was a colonnade (of reused columns) with a length of 200 metres along the right bank of the Fossa Traiana, to the south of the basin of Claudius and as a matter of fact on top of the south mole. Behind the porticus were shops. It was built c. 425 AD in honour of Placidia, the mother of Emperor Valentinianus III. It is the last monumental building of Portus. It was decorated with a statue on a marble base found amongst the ruins in 1822 (Thylander B327). The statue was erected in the years 425-450 AD by Flavius Alexander Cresconius, Praefectus Annonae:
SALVIS D D N NRIS (= dominis nostris)
THEODOSIO ET PLACIDO
P P A A V V G G (= perpetuis Augustis)
FL(avius) [al]EXANDER CRESCONIVS
V(ir) C(larissimus) PRAEF(ectus) ANN(onae) VRB(is) ROM(ae)
AD ORNATVM PORTICVS
Base of a statue from Portus, set up by Flavius Alexander Cresconius,
praefectus annonae, ad ornatum porticus Placidianae.
Now in the Piccolo Mercato in Ostia. Photograph: Jan Theo Bakker
The base was found together with an inscription from the fifth century on part of an architrave (Thylander B328), that also mentions the Porticus.