Entries in the Roman Martyrology record:

1. ✴under 14th April: “ At Teramo, St Proculus, bishop and martyr”; and
   

2. ✴under 1st June: “At Bologna, St Proculus, martyr, who suffered under the Emperor Maximian”.
   

St Proculus (14th April)

The Hieronymian Martyrology records SS Valentine, Proculus, Ephebus, and Apollonius from Interamnae under this date.  This was probably an error: St Proculus should probably appear alone here.  (See below for another St Proculus associated with St Valentine and the other saints).  St Proculus is recorded alone under this date in the Martyrology of Florus, where he is described as a martyr (but not a bishop) from Interamnae. 

In the bull by which he restored the diocese of Terni in 1218, Pope Honorius III recorded that the Duomo had previously belonged to the glorious martyrs, St Valentine and St Proculus and to the confessor, St Anastasius.  SS Valentine, Proculus and Anastasius remained the patron saints of Terni until 1644, when Pope Urban VIII. ratified the decision that St Valentine alone should have this designation.

The list of  bishops of Terni in the website of the Diocese of Terni, Narni and Amelia  contains two early bishops of this name:

1. ✴Bishop Proculus I was consecrated  in 304 and martyred in 310.  He is succeeded in this list by St Volusianus (310-20) - see below.
   

2. ✴Bishop Proculus II, a Syrian was consecrated in 533 and discharged in 542.
   

There is no record here of a bishop of that name in the reign of Pope Eugenius I (see below).

St Proculus (1st June) and the legend of the Twelve Syrians

The Hieronymian Martyrology records St Proculus at an unspecified location on 1st June.  This entry also appears in the Martyrology of Bede, but with the location specified as “Interamne sive Narniis” (Terni or Narni).   Another entry in the Martyrology of Bede under 6th June records: “Et Interamne, translatio corporis beati  Proculi, martyris” (the translation of the relics of the martyr, St Proculus, at Terni).  It is possible (but by no means certain) that this translation was of part or all of the relics from Terni to Bologna.  There certainly were relics of a saint named Proculus at Bologna at an early date: St Ambrose of Milan sent fragments of them to Bishop Victricius of Rouen in ca. 396, and Bishop Paulinus of Nola received others (probably from Bishop Victricius) soon after.  A leggendari of 883, which probably came from Bologna, refers to the translation of SS Proculus and Laurence on 1st June, but gives no other information.

In fact, two saints called Proculus are venerated at Bologna on 1st June.  Their legends are known from 18th century copies of a codex from the Abbazia di San Procolo, Bologna that has since disappeared: 

1. ✴"Proculus miles m. Bononiae" (St Proculus the soldier, martyred at Bologna - BHL 6954 and 6954b).  He killed the Roman prefect of the city in the reign of the Emperor Justin I ( 518-27) and was subsequently executed.  These events took place when “Ariana pestis pullulabat”, the Arian plague was growing, a reference to the rule of the Goth Theodoric (493–526) in Italy.
   

2. ✴“Proculus ep. (Interamnensis) m. Bononiae” (Bishop Proculus of Terni, martyred at Bologna - BHL 6955-7), who is essentially the figure in the Legend of the Twelve Syrians.
   

In the “classic” version of the Legend of the Twelve Syrians (BHL 1620-2), St Proculus  travelled from Syria to Rome with his extended family in the reign of the Emperor Julian the Apostate (361-3).  The family was imprisoned there and St Anastasius, the pater familias, was beheaded.  St Brictius, one of the two sons of St Anastasius in the group, led the rest of the family to safety along via Cornelia, where they split up.  St Proculus went to “Castrum carsulanum”, which was probably Carsulae, where he became a priest under St Volusianus.  Pope Eugenius I (654-7) called him to Rome to answer unspecified charges, but he was exonerated and returned to Carsulae.

The legend of Bishop Proculus of Terni, martyred at Bologna (BHL 6955-7) is based on this classic version of the Legend of the Twelve Syrians.  However, it is now set in the reign of the Emperor Justin I ( 518-27) rather than that of the Emperor Julian the Apostate (361-3).  It seems that this later date was imported from the legend of St Proculus the soldier.

In the legend of St Proculus the bishop, he travelled to Rome with his companions and then, fearing arrest, went to Narni where he met St Valentine, Bishop of Terni.  They were both arrested but an angel helped them to escape.  St Proculus went to “Laxanum” (rather than Carsulae, as in other versions the legend), where he became a priest under St Volusianus.  When St Valentine died, he became Bishop of Terni, and it was at this time that Pope Eugenius called him to Rome.  St Brictius subsequently called him to Spoleto, and when he faced persecution there, he fled to Bologna.  He was executed there  “Totile Regis imperante, Iustino Imperatore”.  [In fact,  the reign of Totila (542-52) coincided with that of the Emperor Justinian (527-65) rather than the Emperor Justin I].  The details of the death of St Proculus here are taken from the account of the martyrdom of St Herculanus in the Dialogues of Pope Gregory I.

In 1586, the Dominican scholar, Giovanni Battista Bracceschi (referenced below) drew on an copy (1583) of an extract from an account (c. 1474) by Pietro Antonio Nardi of the lives of four saints of Terni: SS Anastasius, Proculus, Valentine and Cyril [beati Cirilli civis Interamnensis, episcopi Hierusalem].   Fortunately, the Bollandists drew on a later copy of the relevant extract, and reproduced it in 1898 (search on “Nardus”).  It had St Proculus and his brother James (whom he claimed as his ancestors) travelling from Damascus to Bologna in the reigns of  the Emperor Justinian and of Pope Hormidas, “qui pontifex fuit anno 516”.   This dating suggests that Pietro Antonio Nardi took the legend at Bologna as his source.  This document was one of the sources for the Legend of the 300 Syrians, a construct by local scholars in the late 16th and early 17th centuries in which a large group of Syrian monks arrived in Umbria in 516.

The account of Pietro Antonio Nardi, Pope John II invested St Proculus as Bishop of Terni in 536, “tempore Totilae, regis Gothorum” (in the time of Totila, the Gothic King).  He and his nephew Proculus II (the son of his brother, James) were considered to be martyrs

Other Possible Doubles for Proculus

SS Proculus, Ephebus, and Apollonius (14th February)

The Roman Martyrology records under 14th February “At Teramo (Terni):

1. ✴St Valentine, bishop and martyr, who was scourged, committed to prison, and, because he remained unshaken in his faith, was taken out of his dungeon in the dead of night and beheaded by order of Placidus, prefect of the city. 
   

2. ✴[and] the holy martyrs Proculus, Ephebus, and Apollonius, who, while keeping watch at the body of St. Valentine, were arrested and put to the sword by command of the consular officer, Leontius”.
   

The Hieronymian Martyrology records

1. ✴under 14th February, St Valentine;
   

2. ✴under 15th February, SS Apollonius, Ephebus and Proculus;
   

3. ✴under 14th April, probably in error (see above), SS Valentine, Proculus, Ephebus, and Apollonius  from Interamna; and
   

4. ✴under 1st May, SS Valentine, Proculus, Ephebus, and Apollonius from Interamna.
   

The martyrdoms of SS Valentine, Proculus, Ephebus, and Apollonius are recorded under 14th February in the Martyrology of Bede.

St Proculus of Narni (1st December)

The Roman Martyrology records under 1st December: “At Narni, St Proculus, bishop and martyr, who, after performing many good works, was beheaded by order of Totila, king of the Goths”.

As noted above, St Proculus was recorded in the Martyrology of Bede, with the location specified as “Interamne sive Narniis” (Terni or Narni).  However, this entry was for the 1st June.  The earliest known reference to this saint on 1st December is in the Martyrology of Usuard. 

The list of  bishops of Narni in the website of the Diocese of Terni, Narni and Amelia  includes a Bishop called Proculus, who was elected in 535, the year of his death.

St Proculus at Ferentium

The Dialogues of Pope Gregory I include an account of Bishop Boniface of Ferentium celebrating mass in his diocese “near the blessed martyr” on the feast-day of St. Proculus.  Unfortunately, we do not know the feast day of this saint.