Other Forms.—a. In the drama from words in -εύς we find rarely -έα in acc. sing., -έας in acc. pl. -έος and -ῆος, -ῆες, -ῆας are occasionally found.
b. The nom. pl. in older Attic ended in -ῆς (βασιλῆς), derived either from -ῆες by contraction or from -έης (once on an inscription) by 34. -ῆς occurs on inscriptions till about 350 B.C., and is the form to be adopted in the texts of authors of the fifth century and in Plato. -έες occurs rarely, but is suspected. βασιλεῖς (regular on inscriptions after 329 B.C.) is from analogy to ἡδεῖς.
c. The acc. pl. βασιλεῖς was not used till the end of the fourth century. -ῆς (the nom. form) is used for the acc. in a few passages ( cross251 b).
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].