Bauman, Crime and Punishment in Ancient Rome p. 23; Millar, "Condemnation to Hard Labour," p. 122 ("the carcer was employed for the retention of prisoners awaiting trial or punishment," but "any tendency for it to be used as a place of sentence was always resisted") and p. 130.
newadvent.org
Maurice Hassett, "Mamertine Prison," in The Catholic Encyclopedia (New York: Robert Appleton Company, 1910), vol. 9, New Advent edition online.
