This five-hundred strong infantry unit was recruited from the Delmatae tribesmen inhabiting the Adriatic coast of modern Croatia. This regiment is attested on a number of inscriptions at High Rochester in Northumberland, forward of Hadrian's Wall, two of them dated to the reign of Antoninus Pius in the period AD138-61, although the unit is well attested in the fort at Maryport in Cumbria during the same period. A tombstone also recovered from the High Rochester fort may be attributed to Cohors I Dacorum.
