Cubiculum of Leonis, Catacomb of Commodilla, Rome. Late 3rd-early 4th century.
Khan Academy Articles and video
Christianity, an introduction for the study of art history
Early Christian Art, an Introduction
The Vatican Website
The Christian Catacombs of Rome
National Geographic Article
BBC Article
The Secrets of Santa Priscilla Catacombs
PBS Article
In the Catacombs: From Jesus to Christ, the First Christians