Decima Gemina

Raised by Octavian by the merger of two legions of Mark Antonys following the Battle of Actium in 31BC. 

In the third century, the tenth legion received several surnames, which suggest its loyalty to the emperors Caracalla or Heliogabalus (211-217 or 218-222; Antoniniana), Gordian III (238-244; Gordiana), Decius (249-251; Deciana), Florianus (276; Floriana), and Carinus (283-285;Cariniana). From this set of rulers, Caracalla, Decius, and Carinus waged Germanic wars. Gordian III is known for a war against the Sasanian dynasty in Persia, and may have used a subunit of X Gemina. Why Florian, who ruled for a mere 88 days and never left Asia Minor, would award an honorific to a unit that was stationed on the banks of the Danube, is unknown.

During the conflict between the emperor Gallienus (260-268) and his rival Postumus of the Gallic Empire, the legion certainly supported the first-mentioned, for which it was rewarded with surnames like Pia VI Fidelis VI ('six times faithful and loyal'). This proves that it was awarded the titles Pia II Fidelis II to Pia V Fidelis V in the 164 years between Domitian and Gallienus.

The tenth, 'twin' legion, like almost all Caesarian legions, the emblem of this legion was a bull. (Some historians believe it may of been raised by Ceasar in 61AD in Nearer Spain)

4th century
At the time of the Notitia Dignitatum, the first detachment of Decima Gemina was under the command of the Magister Militum per Orientem, and was a comitatensis unit.The other detachment was still in Vindobona, under the command of the Dux Pannoniae primae et Norici ripensis.

Cheers

Matt


Literature 
J. Gómez-Pantoja, "Legio X Gemina", in: Yann Le Bohec, Les légions de Rome sous le Haut-Empire (2000 Lyon) 169-190 
J.K. Haalebos, "Römische Truppen in Nijmegen", in: Yann Le Bohec,Les légions de Rome sous le Haut-Empire (2000 Lyon) 465-489 
M.C.J. Miller, "Legio Decima Equitata. The Tenth Legion after Caesar and the colonists of Patrae", in Ancient World 2 (1979) 139-144 
A. Morillo Cerdán & V. Garcia Marcos, "Nuevos testimonios acerca de las legiones VI Victrix y X Gemina en la region septentrional de la península Ibérica", in: Yann Le Bohec, Les légions de Rome sous le Haut-Empire (2000 Lyon) 589-607 
I. Piso, "Les légions dans la province de Dacie", in: Yann Le Bohec,Les légions de Rome sous le Haut-Empire (2000 Lyon) 205-225