schema Alcmanicum

(σχῆμα Ἀλκμανικόν). A name given to the use of a verb in the plural (or dual) number with a noun in the singular when there follows another noun to which the verb has also reference—e. g. in Homer (Il. v. 774): ἧχι ῥοὰς Σιμόεις συμβάλλετον ἠδὲ Σκάμανδρος.

So also Od. xi. 513; Il. xx. 138. The grammarians say that the construction was frequent in the poems of Alcman, whence the name.