Cassiodorus Cassiodorus, Institutiones, 2, 1, 37 In plurali autem genitivo, ablativus singularis formas vertit. Nam in a aut o terminatus, in rum exit, e correpta in um; producta, in rum; i terminatus in um. Dativus et ablativus pluralis a in is exeunt et in bus. Quae praecepta in scholis sunt tritiora; sed quotiens in is exeunt, longa syllaba terminantur; quoties in bus, brevi.