The Gregorian Mass known as the Missa de Angelis is believed to date from as early as the 9th century. It is a plainsong mass, designed to be sung in unison and a cappella. It is also known as Mass VIII. It can be found in our hymnbook as follows:
500 Agnus Dei
Here is a performance of the Mass:
The Kyriale is a collection of Gregorian chant settings for the Ordinary of the Mass. It contains eighteen Masses (each consisting of the Kyrie, Gloria [excluded from Masses intended for weekdays/ferias], Sanctus, and Agnus Dei), six Credos, and several ad libitum chants. This collection is included in liturgical books such as the Graduale Romanum and Liber Usualis, and it is also published as a separate book by the monks of Solesmes Abbey.
In the Kyriale, the individual chants of the Ordinary are grouped into complete sets, whose title usually indicates the opening of the prosula formerly sung to each respective Kyrie melody. These masses are followed by individual items not grouped with the complete masses. Read more here.