Near Rome, central Italy. Ostia, now a small borough, was the ancient port of Rome, the first Roman colony founded by Ancus Marcius, chiefly to exploit the salt deposits. Prior to Imperial times, it had no harbour, the mouth of the Tiber affording the only shelter for shipping; the Emperor Claudius, therefore, built an artificial harbour at Ostia, and Trajan afterwards built a basin there, and enlarged the canal by which the harbour communicated with the Tiber. Here a new city sprang up, called Portus Romanus, which was embellished by Marcus Aurelius and other emperors, and connected with Rome by a new way, the Via Portuensis, along the right bank of the Tiber. With the decay of the Empire, Ostia and Portus decayed, and in the tenth century the basin of Portus had become a marsh. Between 827 and 844 Gregory IV restored the city, fortified it against the Saracens, and gave it the name of Gregoriopolis.

Leo IV defeated the Saracen fleet at Ostia in 847, and stretched a chain across the Tiber. Ostia was afterwards fortified by Cardinal Ugolino (Gregory IX), by Cardinal Giuliano della Rovere (Julius II), and by Paul III, while Paul V, in 1612, reopened the basin north of the Tiber. Excavations at Ostia were begun under Pius VII; they disclosed the forum, a theatre, three temples, the sanctuaries of Mithra and of the Magna Mater, the emporium, and a great many inscriptions.

Not counting St. Cyriacus, martyr, and Maximus the bishop who, according to the Acts of St. Laurence, consecrated Pope Dionysius in 269, the first Bishop of Ostia was Maximus, A. D. 313. We know from St. Augustine that the Bishop of Ostia sometimes consecrated the pope. St. Monica (q. v.) died at Ostia, and was buried in the church of St. Aurea, though her body was transferred, later, to Rome. The great hospital which St. Gallicanus built at Ostia was a noted establishment. As early as 707, the Bishop of Ostia resided at Rome, holding the office of bibliothecarius sanctæ ecclesiæ. The popes later on employed them in the administration of the Universal Church, especially in legations. They were among the bishops who took turns in exercising the pontifical functions during vacancies of the Holy See, and who became known as episcopi cardinales, or "cardinal bishops". Among the Bishops of Ostia were Georgius, who in 755 accompanied Stephen III to France; Donatus, who was sent by Nicholas I to Constantinople in 866 to deal with the case of Photius, but was stopped at the Byzantine frontier. In 869 this Donatus was head of the legation to the Council of Constantinople and to Bulgaria. Others were: Blessed Gregory (1037); St. Peter Damian (1058); Gerard of Châtillon (1072) and Otho of Châtillon (Urban II) (1077), who served as legates on various occasions, and were both imprisoned by Henry IV; Leo Marsicanus, also called Ostiensis (1101), the chronicler; Lambert Faganini (1117) (Honorius II); Alberic (1135), legate in the Holy Land, where he presided over the Council of Jerusalem, and also in England and France. Hugo (1150) was the first to bear the double title of Ostia and Velletri.

Velletri (Velitræ) is an ancient city of the Volscians, which, in 494 B. C., became a Latin colony, but revolted in 393, and was among the first of Rome's enemies in the Latin War, for which reason, in 338, the walls of the town were destroyed, while its inhabitants were taken to Rome to people the Trastevere, their lands being distributed among colonists. Velletri was the home of the family of Augustus. In its later history, the battle of Velletri (1744) is famous. The cemetery near the Villa Borgia shows the great antiquity of Christianity in this region. The first known Bishop of Velletri was Adeodatus (about 464); Joannes, in 592, was entrusted by Gregory the Great with the care of the Diocese of Tres Tabernæ (Three Taverns), now Cisterna (see ). From the eighth century, Velletri again had bishops of its own; of whom the last recorded was Joannes (868). Another see, united with Velletri, is that of Norma (Norba); its territory is a deserted, malarial country; only one of its bishops, who lived in the tenth century, is known. Other bishops of Velletri, before the union of the sees, were Gaudiosus (Gaudericus), one of the legates to the Council of Constantinople (869), and Joannes, who, in 1058, usurped the pontifical Throne, under the name of Benedict X.

Among the successors of Hugo in the united sees were Ubaldo Allucingoli (Lucius III); Ugolino de'Conti, 1206 (Gregory IX); Rinaldo de'Conti (Alexander IV); Petrus a Tarantasia, O.P., 1272 (Innocent V); Latino Malabranca Orsini (1278), a great statesman and diplomat; Nicolò Boccasino, O.P. (Benedict IX); Nicolò da Prato, the pacifier of Tuscany (1304). During the Avignon period, all the bishops of Ostia were Frenchmen, residing at Avignon or serving as legates; the most famous of them was Pierre d'Etain (1373), who persuaded Urban V to go to Rome. During the schism, each of the rival popes appointed a Bishop of Ostia. Among the legitimate bishops may be mentioned William of Estouteville (1461), who built the episcopal palace; Giuliano della Rovere (Julius II); Alessandro Farnese, 1524 (Paul III); Gian Pietro Carafa, 1534 (Paul IV); Alessandro Farnese (1580), who restored the cathedral; Antonio M. Sauli (1623), founder of a Basilian monastery; Domenico Ginnasio (1683), who restored the cathedral and founded a hospital at Ostia; Bartholommeo Pacca; Louis Micara (1844).

The united dioceses have 16 parishes, with 34,000 inhabitants, 5 religious houses of men and 5 of nuns, 1 educational establishment for male students, and 3 for girls.

CAPPELLETTI, Le Chiese d'Italia, I; BORGIA, Istoria della Chiesa e città di Velletri (Nocera, 1723).

U. BENIGNI