a. The ending -ας is produced by adding νς to the stem (ν becoming α between two consonants by 35 b). Thus φύλακ-ας is from φυλακ-ς. This -ας may be added even to ι and υ stems: Hom. πόλι-ας, ἰχθύ-ας, Hdt. πήχε-ας. Hom. πόλι_ς is from πόλι-νς (Cretan).
b. The nominative pl. masc. or fem. is sometimes used instead of the accusative pl.: τριήρεις 264, πόλεις and πήχεις 268.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].