What is the female equivalent of a marquess in British nobility?

Tremain White, you can be such a know-all sometimes Tremain.

Peerage titles do still exist; and there is even a book of peerage available, within the Queen's realm -- so that one (or, one who ought be 'in the know'...) may check same.

Player #53553731, I'm pretty sure these peerages still exist. But they don't wear the clothes in that picture any more!