Late Roman Unit Sizes

[Longovicium]

Perseus details the Mattiaci as follows:

http://www.perseus.tufts.edu/hopper/text...ttiaci-geo

MATTIACI

Eth. MATTIACI a German tribe, perhaps a branch of the Chatti, their eastern neighbours, probably occupied the modern duchy of Nassau, between the rivers Lahn, Main, and Rhine. They are not mentioned in history until the time of the emperor Claudius; they then became entirely subject to the Romans (Tac. Germ. 29), who built fortresses and worked the silver mines in their country. (Tac. Ann. 11.20) In A.D. 70, during the insurrection of Civilis, the Mattiaci, in conjunction with the Chatti and other tribes, besieged the Roman garrison at Moguntiacum (Mayence: Tac. Hist. 4.37); and after this event they disappear from history, their country being occupied by the Alemanni. In the Notitia Imperii, however, Mattiaci are still mentioned among the Palatine legions, and in connection with the cohorts of the Batavi. The country of the Mattiaci was and still is very remarkable for its many hot-springs, and the “Aquae Mattiacae,” the modern Wiesbaden, are repeatedly referred to by the Romans. (Plin. Nat. 31.17; Amm. Marc. 29.4; AQUAE MATTIACAE) From Martial (14.27: Mattiacae Pilae) we learn that the Romans imported from the country of the Mattiaci balls or cakes of soap to dye grey hairs. The name Mattiaci is probably derived from matte, a meadow, and ach, signifying water or bath. (Comp. Orelli, Inscript. Nos. 4977 and 4983; Zeuss, Die Deutschen, p. 98, foll.)

So it would seem that the Auxilia is perhaps named after this tribe which in turn is possibly named as those by the' water meadows'? I speculate obviously!