Stems in sigma are contracted where ς falls out between the vowel of the stem and the vowel of the ending ( cross120). Thus, γένος
a. The masculine and feminine accusative plural, when it is contracted, borrows the form of the contracted nominative plural. -εις is not derived from -εας. In the dative plural the union of ς of the stem and ς of the ending produces σς, which is reduced to ς without lengthening the preceding vowel ( cross107).
b. Masculine stems in ες with the nominative in -ης are proper names; the feminine τριήρης
c. Neuters with stems in ες have -ος in the nominative, accusative, and vocative singular; neuters with stems in ας have -ας in these cases.
d. Some stems in ας have also a stem in ατ or α_τ ( cross258).
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].