Complex sentence: Marchi collected the sculptured monuments of the early Christian ages, while de Rossi the ancient Christian inscriptions; a third department of the museum consisted of copies of some of the more important catacomb frescoes.
Split into simpler sentences: Marchi collected the sculptured monuments of the early Christian period, while de Rossi the ancient Christian inscriptions. A third department of the museum consisted of copies of some of the more important catacomb frescoes.