Why is the sh sound written with a c?

soft-cThe second c of coriaceous comes before an e, which makes it soft. Combined with the vowels behind it, it produces the sh of the ending.

heard ascoriaseous

spelledcoriaceous

saidko-ree-AY-shus

coriaceous, with a c before the ending.

This is one of the quieter oddities of English spelling. A soft c followed by a weak vowel and another vowel turns into a sh: ocean, gracious, herbaceous, coriaceous. The s is doing an honest job of the sound and destroys the visible link to corium, which is where the c comes from.

Also on this leaf

coriaseus corriaceous coriaceus coreaceous ceriaceous

All six are on the same page. All 6 spellings of coriaceous.