During the reign of Constantine I (the Great, 306-337) the popes
were: Marcellus I (308-309), Eusebius (309-310) Miltiades
(311-314), Sylvester I (314-335), Mark (330), and Julius I
(337-352).
Between 304 and 308 there was an interregnum, a period where
there was not a pope.