134. Stems in -ευ, -αυ, and -ου (§ 136, below) lose υ before a vowel (§ 27; cp. Latin nāvis and bōs, bovis).
1. in the lengthening of the endings -ος, -α, -ας to -ως, -ᾱ, -ᾱς;
2. in the older nominative plural ending -ῆς (contracted from -ῆες); open -η- later became closer -ει- (§ 24).
b. In poetry, less often in prose, the accusative plural is sometimes like the nominative, in -εῖς.
Πειραιέως οr Πειραιῶς
Πειραιέᾱ οr Πειραιᾶ
Εὐβοέων οr Εὐβοῶν
Εὐβοέᾱς οr Εὐβοᾶς