Modern Hagiography
Sanctorale Catholicum – Saint Cletus.
Lives of the Saints by Alban Butler.
Lives of the Saints by Sabine Baring-Gould – Pope Cletus & Pope Marcellinus.
The Liturgical Year by Dom Guéranger.
Our Birthday Bouquet Culled from the Shrines of the Saints and the Gardens of the Poets by Eleanor C. Donnelly – Pope Cletus.
Pictorial Half Hours with the Saints by Auguste François Lecanu – Pope Cletus.
The Lives of Saints, with Other Feasts of the Year, According to the Roman Calendar by Peter Ribadeneira.
Tradition in Action – Pope Marcellinus
Medieval Hagiography
Pope Cletus – BHL & Supplement
Pope Marcellinus – BHL & Supplement
Acta Sanctorum – Pope Cletus & Pope Marcellinus
The Cult of Saints in Late Antiquity Database – Pope Cletus & Pope Marcellinus
Liber pontificalis – Pope Cletus & Pope Marcellinus
The Golden Legend – Pope Marcellinus and Illustrations
Breviarium Romanum – Latin & English.
Writings of Pope Cletus
Writings of Pope Marcellinus
Art & Architecture
Index of Medieval Art – Pope Cletus & Pope Marcellinus
– El Escorial, Real Biblioteca, Cod. Vitrinas 17, folio 4v: SANCTVS CLETVS EPISCOPVS ET MARTYR TERCIVS VRBIS ROMAE A SANCTO PETRO (detail above)
– Folio 5v: SANCTVS MARCELLINVS XXX PAPA (detail above)
Catalogo generale dei beni culturali – Pope Cletus & Pope Marcellinus
Portail Biblissima – Pope Marcellinus
Vatican Colonnade – Pope Marcellinus
Warburg Institute Iconographic Database – Pope Cletus & Pope Marcellinus
Post-Biblical Saints Art Index – Saint Cletus & Saint Marcellinus
Nihil sub sole novum 