Coins of Constantius II

Can any numismatist kindly inform me by what marks the coins of Constantius II., the son of Constantine the Great, are distinguished from those of Constantius Gallus, his nephew? Mr. Akerman, in his *Rare and Inedited Roman Coins*, gives the following titles as common to both, but does not afford any rule for appropriating their coins:— CONSTANTIVS. NOB. CAES. FL. IVL. CONSTANTIVS. NOB. CAES. D. N. CONSTANTIVS. NOB. C. D. N. CONSTANTIVS. NOB. CAES.