Centurion Rank

[D B Campbell]

I would say senior centurion of the hastati...suggesting, perhaps,the third maniple of the cohort.

I would only add that I think Caesar's use of primus here implies the First Cohort, to which Fulginius had been promoted ex inferioribus ordinibus ("from the centuries below it") -- ordo is another word for centuria. As hastatus prior (presumably), Fulginius is effectively third-highest centurion of the Fourteenth legion.