if you Wikipedia "inverted catenary arch" you don't seem to find an alternative name for it. But you do find CTESIPHON described as the finest example of one. Could this be what you want.
You two are brilliant. I'd found the word, but not made the link to the arch. It's a bad clue (Ctesiphon is a city, not an arch). But I won't forget it in a hurry.
You're absolutely right, of course. I went through every instance of c*e-i---- in Chambers, from caecilian to cretinous. I couldn't find any nine-letter cue->, cwe->, cye-> or cze-> - but overlooked cte-> - do you blame me?