DECUMARIA, in botany: A genus of the monogyne order, belonging to the dodecandra clas of plants; and in the natural method ranking under those of which the order is doubtful. The calyx is decapetalous, superior; the petals ten; the fruit unknown.

DECUMATES AGR., tithed fields, or granted on a tithe, as appears from Tacitus, to that rabble of Gauls who succeeded the Marcomanni, that had till then proved a check to the Roman conquests up the Rhine; and hence probably their name, people living on the marches or limits of the empire. In Cicero we have Ager Decumans, which is of the same import with the Ager Decumas of Tacitus.