Heard spelling

jynoesium

Spelled gynoecium. two changes at once

Why is it c and not s in the middle of gynoecium?

Because the syllable is Latin -cium, not English -sium.

A c before i or e goes soft in English and in Latin alike, so -cium and -sium are the same sound with different histories. Here the history is oikos, whose k was hardened into a c by Latin scribes and then softened again by the i behind it. Jynoesium hears the soft g and the soft c correctly and spells neither.

Say

guy-NEE-see-um, and jy-NEE-see-um is heard just as often

Letters

G-Y-N-O-E-C-I-U-M