ALL Roman historical writers use a naval terminology based on military parlance. Ships on the move in line ahead are an agmen navium, positions in the line are ordines , ships in a single file are said to move tenui agmine—in two columns are said to be birds in ordine navibus. In one passage the three posts in the column are described as prima navis, media classis, extremum agmen. To bring up the rear is agmen cogere.