Epistolarium

Epistolarium The office of the Holy Communion was in the early ages of the Church contained in four volumes, viz. the Antiphoner, the Lectionary, the Books of the Gospels, and the Sacramentary. The second of these, the Lectionary, was the book of the epistles read at mass (Du Cange, Glossarium, s.v. Lectionarius), generally called the Epistolarium, also Comes and Apostolus. — Procter, On the Book of Common Prayer, page 9.