An important study of medieval canon law. Originally published: Graz: Leuschner & Lubensky, 1870. lxx, 981,  pp. Reprint of the only edition. Volume I (all published): Die Rechtssammlungen bis zur Mitte des 9. Jahrhunderts. Inspired by Savigny's great Geschichte des Romischen Rechts im Mittelalter (1815-1831), Maasen's study has a similar organization. He intended to produce a work in five volumes, but only succeeded in publishing the first, which covers the period spanning the end of the Roman Empire to the later decades of the ninth century. Outdated in certain points today, as one would expect, it remains a standard title in bibliographies devoted to post-medieval legal history and canon law.