Among the liturgical books of the Church, the missals and breviaries hold the first place. In earlier times these were differently arranged and had different names from those which they bear at present. The collects and psalms in particular employed at the mass were not included in one volume as they are now, but were taken from a number of books. The essential prayers of the mass were contained in the so-called _Sacramentarium_, in which were also to be found the collects, prefaces, and, in certain cases, even the whole canon. The lections from Holy Scripture, the Epistles and Gospels, were collected together in the Lectionaries, either all together, or the Epistles and Gospels in separate volumes; another name for this kind of book was _Comes_. The psalms and other portions to be sung by the choir were contained in separate books, the Antiphonaries, Graduals, and Hymnaries, etc. It was only in the Middle Ages that for the sake of convenience all the different parts of the mass were contained together in one volume, called a _missale plenarium_, but the separate volumes still continued in use for certain occasions. In the course of our investigations, we have often had to refer to the so-called sacramentaries, of which the three most ancient belonging to the Roman Church are also the most important. These are:— 1. _Sacramentarium Leonianum._ This is a collection of older formularies which had been already drawn up by an unknown author, some of which may well be the work of Leo the Great; the title, which gives rise to the opinion that Leo was the author of the book, is due to the first editor. The only existing codex is in Verona, and is unfortunately imperfect, being deficient in all that relates to Lent and Easter. Since it contains a prayer for Pope Simplicius († 483), Rome must be regarded as the place where it was drawn up, and where it was used, while the date when it took its present shape was the pontificate of Felix II. (483-492).